why does it appear that, for most genera of dinosaur, there is only one described species?
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A lack of fossils, most likely. We don’t have enough evidence to determine what is variation within a species and what constitutes different species. For example, there’s a theory right now that there are actually [three species of Tyrannosaurus](https://www.sci.news/paleontology/three-tyrannosaurus-species-10590.html) rather than one, but it’s contentious because the sample size is so small.
often a lot of time speciation is mostly identifiable via phylogenic traits. considering the wear of time often minute physical difference will be lost in the fossils. I do believe there was one British scientist recently who looked closely at a lot of T-Rex statues in museums and tried to propose they were in fact three different sub species. it was met with mixed reviews.
It’s a matter of available data. If you want to tell the difference between two skeletons, there are not many features to compare.
Now if you compare two living organisms, you can see the color, you can see its behaviors, you can see what it eats, and you can see if they can breed with one another.
Fossilized skeletons tell a story, but the story is very incomplete.
Fossilization happens very rarely. It takes quite specific circumstances to happen. It’s just that dinosaurs roamed the earth for over 100 million years and there were plenty of opportunities for it to happen in that time.
On the other hand, not a lot of any given species gets fossilized, it’s just such a rare thing.
So a small number of fossils, and we definitely haven’t found them all either.
On top of that, often only part of the dinosaur will actually get fossilized. It wouldn’t be surprising if we found a random dinosaur foot because the rest of the animal got eaten and the foot got left behind. Or other random bits, or broken parts that got left behind.
The rare occasions where we find a whole dinosaur are because it got swallowed up in a mudslide, or fell into a peat-bog.
A dinosaur that keels over and dies of natural causes is probably not going to get fossilized.
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