FLES was my go-to, but they've disappeared so I've been expanding my horizons. However I'm seeing that being under 30 is an exclusion criteria for many providers... why is that? Higher risk?

In order to determine whether or not the lady in question and you will have a great time together.
My guess would be sometimes providers find younger guys disrespectful, flaky, too aggressive and trouble but, that can happen in any age group. If you have great references, you are willing to undergo screening and know how to treat the ladies (respect and consideration are quite important factors on this) scheduling a date should not be a problem..

Some providers and agencies prefers clients with only 9&10 reviews. They don't want their average to drop. I was recently declined for a booking as I have given a well known provider 8s in the past.