Extinction events are rare. For all practical intents and purposes you could just say that body types change slowly, period.
You could also say that changes in body types appear suddenly (e.g. in the fossil record), though rarely. Same thing.
Whether such evidence supports an evolutionary process or an external bias is another thing entirely, however.
New body types in the fossil record are almost always associated with extinction events or geological events that produce isolation.
Breeding experiments by humans demonstrate that physical size and shape can be modified in decades rather than mellennia. The key difference between rapid and slow change is selection and changes in the conditions that determine reproductive success.
There are genomes that are more plastic than others. Dogs can be shaped rather easily, but cats have pretty much the same body shape. The answer to why this is will be found by research rather than by speculation.