It's pretty much a combination of the money, plus the prestige (playing for Man U has a lot more cachet than QPR). But the whole EPL is pretty much like OPEC, and relegation is essentially tantamount to economic death for teams that don't manage to claw their way back up the following season (as West Ham and Newcastle both did). Some teams (e.g. Leeds United) just go into freefall after being relegated...I think they went all the way down to League 1.
They did (2007); back in the Championship now.
Last season Bradford City (a Premiership club briefly from 1999-2001) just missed becoming the first former Premiership club to go out of the Football League altogether, and Coventry City (in the first flight continuously from 1967-2001) are faced with a winding-up order from the revenuers, over unpaid stadium rent.
When they fall, they often fall hard.