Because this season is done with. No domestic cup to go for, no real hope of the UEFA Cup (if we're being realistic) and Europe through the league is a pipe dream with our away form combined with remaining away matches. January has gone so there's no money to spend until the summer. Bring in another manager now, he can see out this season safe in the knowledge that any failure to qualify for Europe next season will be blamed on Hughes and that he can assess the squad fully before getting the money to spend in the Summer.
In short, whoever we bring in can't possibly have City putting in such terrible performances as today and Stoke, there's nowhere to go but up from those games.