Because Rafa proved he is a brilliant manager with not many funds, as he showed at Valencia and here. And any manager we get is very unlikely to a) win the FA cup or b) win the European cup in his first 2 seasons. Let alone getting anywhere in the league.
Well according to Matt Le Tissier, Benitez is only bothered about the Champions League because "it looks better on his CV" so he can move "to a bigger and better job". Hmm.