Williams just doesn't have the money anymore. Their disastrous relationship with BMW made the team a poster child for Spanky's campaign to reduce F1's dependence on manufacturers. Left without a top tier engine before (arguably during) BMW's tenure, Williams slid in competitiveness and I suspect their budgets began to shrink.
With McLaren losing their Vodaphone sponsorship -- and soon to lose their Mercedes engines -- you can see the similarities. Looking for a principal sponsor while struggling on the track is not a good place to be.
It has been said that man is a rational animal. All my life I have been searching for evidence which could support this.
-- Bertrand Russell
To make an apple pie from scratch,
you must first invent the universe.
-- Carl Sagan