Because the slopes are too shallow.
Aqua Virgo, which is still in operation and is what to this day is the source for the water in the Trevi Fountain, is 20.5 km in length and has a total drop of four meters. That means its average slope is 0.02%. As in 7/10ths of one inch of drop per 100 yards of viaduct. You wouldn't build up much of a head of steam on anything that shallow.
Which was part of the point. Fast-flowing water would have accelerated erosion. They built the aqueducts to last. Most of the rest were demolished for their stone but Aqua Virgo has lasted (with infrequent updates and repairs) for 2040 years.
What's that old saying, "They don't build 'em like that any more!"
