Ferguson and his love for power and control.
If the Glazers sack Moyes they risk upsetting Fergie. If Ferguson turns on them, he'll probably let the world know the truth about the finances.
I suspect the Glazers don't want United fans to up the protests and make life difficult for them, this could affect the share price. Especially since the sponsors won't want such bad press.
I have a feeling it was not lack of value in the market that stopped Ferguson spending early on in their reign but the need for our transfer budget to be used instead to service the debt. Perhaps if they don't do as he says then he will tell the world how they came so close to crippling the football club.
They care only for the share price so I think if Ferguson wants Moyes to be given time then that's what he'll get. Ferguson in turn probably won't sack Moyes because he'll look like a hypocrite.
So basically the best thing for all parties involved is to help Moyes turn things around.