Because it's wrong to vote against Alito. We don't need Chafee. The Senate is structurally favorable to the GOP... it's essentially impossible for Democrats to retake and hold it.
The Senate GOP has sucked and Chafee is part of the problem.
Could you make a map of what you think a Senate would look like if the States voted along the 'structural' lines of which you spoke? I attempted to do so, and came up with this (gray being one of each party). I'm not agreeing with your thesis, I just find it interesting.