I for one don't think it was ever about life on other planets, it was always plunder on other planets. Wait, that would be a great title for a B film :)
Well, there’s really no plunder out there, or at least none that we have found that is worth hauling back. So I think at some point, the bureaucrats decided to shift the mission to something that isn’t as easily quantified, like colonization, searching for habitable planets in other solar systems, etc. Those are much easier to sell to Congress than a program to get us resources that doesn’t produce any resources.