Presumably because, even according to the “Modern Orthodox” (as Rabbi H. Schachter stated) you need to live where you will be the best Jew that you can be (and for your family as well).
So if, by moving to Israel, you become CH”V a Zionist (to any extent) then it’s probably smarter to avoid becoming an oveid A”Z and not move there.
Then there are also the issues of parnassa and family, elderly relatives, etc.