Why is Kayla Harrison not in the UFC?

2x Olympic Judo gold medalist, and currently 15-0 record in MMA. I feel like if the UFC signed her she'd be a guaranteed top contender, if not already champion material... I just don't understand why she isn't already signed, is there any backstory there? The only problem I see is that she currently fights at 155lbs, which isn't a weight class which exists in the UFC