What is it about the geometries that makes a time trial frame specific? Thanks.

Seat tube angle for one.

Usually a steeper seat tube which positions you more over the BB, and b/c you're more forward, it's easier to get to the aerobars.

Also typically a shorter head tube (to get the front end of the bike lower), and shorter chainstays.

In general, your position on a TT/tri bike is farther forward in relation to the bottom bracket, so just about everything else on the bike needs to be adjusted to compensate for that, otherwise the bike would be squirrely and hard to deal with. (Which is precisely why a road bike with bolt-on aerobars and a forward set seatpost results in a poorly handling bike.)

Thanks all. Are the top tubes generally shorter as well?