Because FOX News is not a news station. They are a business. Yes a business. The guys at corporate hire the most outlandish of hosts (Beck, Hannity, O'Reily) not in order to give you the news, but instead to get viewers which gets them more sponsorship, and with more sponsorhip, more money. you can also look at it in the perspective of their network station FOX. Their comedies like Family Guy and The Simpsons go where not many comedies have gone before which attracts viewers and sponsors.
If you look at FOX news and FOX the network, the news station is clearly leans to the rights, while the cable network leans to the left because of their lack of values in comedies. The FOX corporation only cares about getting viewers. If Tiger Woods and lack of family values attracts viewers then they'll keep at it because no other news station will. When other news stations are hounding Ensign and Sanford they find another risque or controversial story to stay on top.
And the sad thing is, is that this scheme tends to work pretty well for FOX