OK, the slightly longer version.
There is no money for new defence aircraft, especially replacing one which has just been cut.
Further details.
If priorities are readjusted with no change in threat:
Whoever cut Nimrod now looks like an idiot.
Anyone who admits their bit of Defence could be cut in favour of a new MPA knows they will find their bit cut, and there'll still be no MPA.
Or, the longer version
Will a new MPA help the current government get re-elected better than other things they could spend that cash on? No