It's very simple: as others have said, it's because Obama is for it.
The individual mandate was first promoted by the Heritage Foundation, a far right wing think tank. The republicans in Congress were for it. Well, they were for it before they were against it. And the only reason they are against it now is because Obama suggested it. If W. had promoted it, they would be for it.
Also, the idea that it's unconstitutional is absurd. Freakin' George Washington forced every household to have a gun. Washington! The commerce clause clearly allows the government to do this. And not only that, but it's not like automobile insurance. For instance, I don't drive, so I don't have auto insurance. That doesn't cause anyone else's insurance rates to rise. With health insurance, however, every single person will have healthcare costs at some point. And if people don't have insurance, it causes the rates for those who do to increase.
Healthcare is a sui generis situation.