People aren't interested in inverts, or in having taxonomic diversity in zoos. Zoo Tycoon buyers want the ABCs and some wacky looking things that they've not seen. Whilst I know that Zoo Tycoon 2 catered for mods to give people the freedom to do whatever they wanted, it didn't include place-able small inverts (the invert and marine houses / standalones being exceptions). So if the 1,000,000 insect species were actually put into the game, they would largely be completely unused. This is to say nothing of the game dev time required to actually build that many different insects and code them individually.
Even if this was dropped to a 'realistic' 100 insect species in the evenly proportioned zoo, there would then only be 1 bird species and a couple of fish. Maybe a single mouse species.
I know this is puritanical, but it's also thinking about where to draw the line. I'm all for increased diversity in zoo sims (heck and in games in general), but I just don't see it happening. I don't think any player actually benefits from having nine agoutis or lions, especially when they are reskins with no thought put into their diverse ecology.