BA, Virgin, EasyJet, Ryanair, Jet2, Monarch, Thomson, Thomas Cook, Flybe and BMI Regional all operate from Manchester and is the only airport in the UK where all of these airlines operate.
Year round based yes but that would only be really true if we forget Glasgow and Edinburgh, EDI with VS short haul only, though both are about to lose their
huge BMI Regional operations shortly.
Blunt answer is that there isn't enough business to keep all the airports open.
There never was, like a lot of what has happened from 1997, and even since the coalition, planning too often remains in the realm of fantasy.