Because endowments matter so much to athletics. Its not like anyone is sharing their endowments within conference.
Also, UCF is ranked 173 by USnews rankings for national universities. Okie State 145. USF 161 WVU 168, Texas Tech 156. A good number of UCF's programs are ranked in the top 100. We're not talking a huge dropoff from universities already in the big 12. Besides, why do we pretend this is about anything but athletics anyways. We're not the big 10.
And no, UCF doesnt have a ton of endowment or fanbase built compared to all in the big 10 but they didnt start building their alumni base until the last couple decades (the school itself being less than 50 years old). It will happen with time, and with the fact that they *have* had at least some athletics history and a strong showing in attendance for their level of competition (bringing 35-45k routinely right now, with a 45k capacity stadium) it shows they could transition well to a higher level. When ashford is doing that then yeah, lets talk about them.
So if UCF has the potential to be a **** hot sports school (we disagree there), why not wait for it to happen on its own before adding them to the conference? Since when do P5 conferences invest in "project schools" that could turn out to be huge programs? Never, and it's because they'd be stupid to do so. The only reason conferences have added members is out of necessity for survival (Big 12), or because the new members add value for their TV contracts (everyone else). If UCF is a potential juggernaut, let them become that before extending an invite...there's no reason whatsoever to split revenue with a school that's not pulling their weight on the off chance that someday they'll be able to contribute their fair share.