I thought they only prerequisit scientists used for where life is likely to be is wherever water is, due to the fact that seems to be the only common connecting feature of life on Earth - the need for water. Even where there appears to be none; I seem to remember a documentary talking about a species of bacteria that is able to get water from rocks, as mental as that sounds.
But anyway, I'm guessing the main reason is because, due to the vastness of space, they have to narrow the search down somehow and this is as good a palce to start as any, ie places that have the same conditions as other places we know for certain life exists. You have to start somewhere. Once we eliminate the places that have the conditions deemed more likely to contain life as we know it we can then move on to the other palces that, if they had life, contain life in a form that we do not know about and therefore cannot reasonably predict.
So basically, it's coz we know life can exist in certain conditions so it makes more logical sense to start the search where those conditions are met and where we know that life definately could exist rather than in places where we've no idea if it's even possible for life to exist there.