Pain
A simplistic answer would be that they simply make sufficient money at SEN to make it worthwhile, regardless of the disadvantages that you point out. The advantage that SEN has in terms of its popularity with travellers and large local catchment also helps.
It was interesting that the introduction of the third daily SEN-AMS was extremely successful so it doesn't appear that SEN's growth potential has yet been reached. I wouldn't rule out further easyJet expansion in the future.