It does. It's called Invicta.
In the UFC you mean?
Because that's dumb. They don't do events based only a few weight divisions. The logistics of booking only one or two divisions makes it impractical. There are three WMMA divisions in the UFC, and it's really more like two and a half.
There simply isn't the depth in any division to do this with any regularity and especially so in WMMA. There are a few great fighters, a bunch of good ones, a whole bunch of mediocre scrappers. Kinda like male divisions, but moreso.
The UFC could make an all WMMA card and it would be fine. I'd watch, and so would most MMA fans, if the card was good enough. They could also make an all Heavyweight card. I assure you the all WMMA card would be more entertaining on average, especially the prelims.