Ryanair operated x4 daily BFS-LGW during the summer, and x5 daily during the winter. Their BFS-STN operation is x3 for the summer, and the winter frequency is still unknown. Losing just one daily frequency isn't exactly a huge drop, especially given that they'll be competing with easyJet on the route.
It also makes more sense for Ryanair to operate the route from STN. They only fly to 4 destinations from LGW, and only commenced the route because the slots were made available as part of IAG's takeover deal of Aer Lingus. It was widely speculated at the time that Ryanair would eventually axe LGW in favour of STN once they re-established themselves in the NI market.