Because if Clinton is seen pouring resources into yet another state and then losing it by twenty points, it will look terrible. She may believe that the risk of this happening is greater than the reward to be gained by trying to keep down Obama's margin in Wisconsin (or even playing to win the state). By not campaigning, she lowers expectations to the point where an Obama victory is less meaningful, as she might say about Louisiana or Maryland, but, if, as is realistically possibly, she comes within ten points without campaigning, it's a nice "moral victory" for her.
She does need every delegate she can get, but is the risk of campaigning in Wisconsin greater than the reward?
I cannot imagine another reason they would not be trying to fight in Wisconsin. But this tactic seems weird coming so late in the process. OH/TX/PA are less than half the remaining delegates. That is a lot looking at it as 3 states out of 15-20, but it is not a lot in terms of accumulating delegates. Will moral victories elsewhere help make real victories in OH/TX/PA big enough to make up a perhaps substantial deficit?
Can the Clinton camp really hold that as long as they win ~15-20 (and this point only just a handful) of the right states that people should stay with them? Money has to be some issue. They have been saying it is all about the delegate count, yet now they seem to be very selective about their targets unless they really believe winning "their" states will pull more delegates then trying to grab whatever from wherever.