I would say it is 35 mn ride on the ATR with 20 mn check in prior to fly compare to 40 mn with FR so altogether you save time.
FR is targeting a different kind of Cutomers the one who want to travel for a cent and are ready to accept the FR terms and conditions
And anyway i believe ryanair is getting an aircraft nearly every week so they need them to fly on any kind of route that could bring a bit of money.