It's a combination of things.
First there weren't a lot of viable options who hadn't won the Rumble before, and while they could have had someone be the first to double up, in the women's division they're obsessed with giving people their due and giving everyone a chance to shine. So, if you take out those who have already won it, fee women have a stronger case than Bayley.
There is also the breakdown in damage ctrl and the match with Lyo they've been building to, and this will bring that about. They didn't need the Rumble for it but it's the only women's storyline they have anyway, so why not?
Also, she got the win because, again, they want to give participation trophies to women who have been around for a while. Bayley was the Janetty of the horsewomen but perhaps with Sasha gone they'd like Bayley to eclipse Banks over time.
