A lot of reasons. It drains 135 which is already not deep enough, it sets a whole division up for void if Cyborg retires or pops again or something, it devalues UFC titles which is already an epidemic in the sport, it ties your hands in terms of matchmaking because while there might be fun matchups for Cyborg they might be against people who simply don't deserve a shot at UFC gold, it requires that every fight in the division be a 5-round title fight which is absurd, etc..

At the end of 219, Rogan said that when there's a dominant champ a division builds around her, but that isn't happening with Cyborg and there's no evidence that it will happen any time soon. That shouldn't stop the UFC from being in the Cyborg business and preclude us from seeing fun fights at W145. But this title shouldn't exist until there's a division to be a champion of. Put another way:

Where’s the harm in having one very good and dominant champion who fights willing contenders?

They're not contenders. They're game, they're awesome, and they come to fight. But Evinger was 0-0 in the UFC, Holm was 1-3 in her last 4 and 0-1 at FW, and Anderson is 0-0 in the UFC. In a real division, they wouldn't be getting title shots. I'm not saying I'm not happy to see them fight Cyborg, I'm saying it shouldn't be for gold.

Have there been any women's FW fights in UFC history other than the 3 title fights? Does that even count as a division?