Because conservatives tend to nominate Judges that say things like this:

Not a single person has voted for me and if we don’t like what the people in Congress do we can get rid of them, you know, if you don’t like what I do, it’s kind of too bad. And that is to me an important constraint. It means that I’m not there to make a judgment based on my personal policy preferences or my political preferences. The only reason, the reason I’m protected from those political pressures is because I’m supposed to make a decision based on the law. And so I don’t think it would be a good idea to turn all the hard issues over to the courts. Those hard issues belong in Congress, they belong in the executive branch. Courts have the responsibility to make sure that those branches abide by the legal limits of the Constitution.

That is a quote from Chief Justice John G. Roberts. It is precisely this type of humility that is needed in the Supreme Court, not activist judges that will suddenly find a right to an abortion, or gay marriage, when none has ever been found before. The full article can be found here.