Who says they aren't? Who could seriously come in?
The likes of EasyJet and Ryanair aren't going to come in anytime soon. Maybe Vueling to Barcelona, Thomas Cook to part base or base, Blue Air to copy the LPL format, Norwegian to go over the pond with a 737 or even the long wanted KLM connection.
Other than that it is hard to see anyone wanting to dilute other bases. With the cargo gains maybe that is the way forward for now.