Because they were in a position where if they continued with Hulkenberg, they were in danger of collapsing. They've lost four major sponsors at once: RBS, who announced they were leaving because they did not weather the recession too well; Philips, who decided against renewing their contract; AirAsia, who moved over to Team Lotus; and McGregor, who toyed with retaining their deal if Williams took on a driver they sponsored - like Giedo van der Garde - but ultimately decided against it. In Maldonado, they get a five-year deal with PDVSA that is not dependent upon Maldonado staying with the team. Even that is not enough to secure their future, because they're toying with sending the company public. Nico Hulkenberg, on the other hand, has repeatedly said that he will not bring any sponsors to the team, or to any other team that offered him a contract. Williams simply couldn't afford to keep him, so thy cut him loose.
Maybe they could have survived with Hulkenberg. Maybe the FW33 will actually live up to what the team have been promising but never delivering since 2005 and actually win a race. And maybe that will be enough to attract new sponsors to the team. But then again, maybe it won't. Frank Williams clearly didn't want to bank thirty years' of history on the idea that a rookie driver might be able to upset the established order in Formula 1. I can't blame him, either. Between Vettel, Hamilton, Alonso, Webber, Button, Massa, Kubica, Rosberg and Schumacher, they are not odds that I'd play.