Because contrary to your assertion, there is indeed abundant evidence indicating that that's what happened.
In the narrow band of the KT (Cretaceous/Tertiary) boundary in the geologic column, dating to 65 million years ago, which separates the era of the dinosaurs from more modern strata in which dinosaur fossils are never found, there is a huge spike in the amounts of the element Iridium, which is normally quite rare in the Earth's crust:
The only known sources of such high levels of Iridium are either materials *deep* in the Earth's mantle, far below the crust, or carbonaceous chondrite meteorites.
That alone would point undeniably towards either a large meteorite strike which splattered its material around the Earth 65 million years ago, or some massive volcanic event originating very deep in the Earth 65 million years ago which spewed deep-mantle material around the planet.
The rest of the evidence settles the matter in favor of the meteorite scenario.
The amount of Iridium in the KT boundary was calculated to be the amount that one would expect from a carbonaceous chondrite meteor about 10 kilometers (6 miles) in diameter.
Later, a 65-million year old meteorite crater was discovered straddling the coast of the Yucatan peninsula in Mexico, 100 miles in diameter and over a mile deep. It was not previously recognized because it had been buried in 65 million years of subsequent sediment, but it was found via seismic scans and gravitational mass scans while looking for oil:
It has been named the Chicxulub crater. Calculations of the size of meteorite which would produce a crater of this size and type resulted in an answer of... 10 kilometers, the same size as a meteorite which would be expected to produce the amount of Iridium found in the KT boundary.
This alone (same size, same timeframe) is very strong evidence that this is the crater produced by the meteorite which produced the Iridium found in the KT boundary. But there's more.
Also in the KT boundary are small glass spheres known as "tektites":
These are formed by meteorite impacts when rock from the impact site is vaporized, and then spreads from the site and condenses back into solid rock in a manner similar to the formation of hailstones from clouds of water vapor. These then fall to the ground (again like hail) littering the ground. Volcanoes can also produce tektites, but only smaller volcanic eruptions, which do not spread the tektites very far. The tektites in the KT layer are found all over the world (in varying amounts, more on that later), effectively ruling out a volcanic origin and again pointing to a large meteorite impact, which would indeed spread tektites worldwide.
Furthermore, the tektites in the KT layer are of a type more accurately known as basaltic spherules, which are not of the type one would expect from a volcanic origin, but instead are consistent with production via a meteorite impact on oceanic crust -- which again is the site of the 65 million year old Chicxulub crater.
Moreover, characteristic elemental ratios can distinguish between materials of an earthly origin (obviously the case for the results of volcanic action) versus extraterrestrial origins (i.e. meteorites). The ratios of platinum-group elements, as well as the relative abundances of ruthenium, rhodium and iridium in the KT boundary, all match the "fingerprint" of meteoritic origin.
And there's more. Both volcanic activity and meteorite strikes can produce characteristic high-pressure, high-temperature crystalline materials such as "shocked quartz". However, each type of production results in crystals that are detectably different in several ways. The shocked quartz (and other) crystals found in the KT boundary match only the type expected from a meteoritic origin:
Even worse for the volcanic scenario, another form of modified quartz called stishovite has been found in the KT layer, and the pressures required to form this type of quartz are far greater than those produced by any volcanic process -- only a major meteorite strike qualifies.
Also in the KT layer is a huge amount of soot (7000 teragrams) which indicates that a large portion of the Cretaceous forests burned at the time of the KT meteorite impact, as would be expected for the impact fireball and rain of molten debris which would splatter around most of the planet in the event of an impact of that size. And yes, the nature of the soot (isotopic makeup, chemical ratios, etc.) allow the safe conclusion that it was extant vegetation which burned at the time, and not coals or fossil fuels, etc., and not ash from a volcanic eruption. Here's an oceanic core sample showing the soot layer, taken 350 miles east of northern Florida:
Finally, the thickness and nature and distribution of the various types of debris found in the KT boundary at various locations around the world are very consistent with a scenario of a 10km meteorite striking on the Yucatan peninsula at a shallow angle from a southeasterly direction. For example, direct debris is thicker nearer the Yucatan as compared to locations farther away around the Earth, there is a heavier "stripe" of debris arrowing norhwesterly across the North American continent originating from "ground zero" in the Yucatan, etc. Furthermore, the pattern of tsunami debris around the Gulf of Mexico and the Atlantic Ocean in the KT boundary matches the direction, distances, and amounts one would expect for a 10km meteorite strike in the Yucatan 65 million years ago, as in this layer of breccia (jumbled, shattered rock) found on the KT layer in Falls County Texas, due to massive tsunamis carrying debris a hundred miles inland:
The rust-colored layer which the pick is resting upon is the tsunami debris, and above that is a thin layer of finely powdered Iridium-rich meteorite strike debris, which settled as dust on the material left behind by the tsunamis.
Furthermore, there is a striking difference in the fossils found below the KT boundary versus above. Over 90% of the species which have fossils appearing below the KT boundary (including the dinosaurs and many others) vanish at the KT boundary. Immediately above, the KT boundary is only a select few of the species found below the KT boundary, then as you look higher above the KT boundary life is seen to be slowly springing back from the mass extinction.
I'm sorry, what's that you were saying about there being "no evidence" for a meteorite strike?
when in fact, the Biblical Flood of Noah offers a much more compelling answer as to why they died out?
As soon as you can figure out how most of the Earth's forests can burn during a worldwide flood, and how other signs of intense heat (tektites, shocked quartz, etc.) are consistent with a flood, and how a flood would produce Iridium and platinum levels only consistent with an extraterrestrial origin, do let us know. Meanwhile, a global flood would leave worldwide evidence as obvious and striking as that left by the Chicxulub meteorite, but no such evidence exists.