umm... because in the past, the many many instances of American support for tyrannies, monarchies, etc. (especially because they have oil) like the Shah, Saddam, the Saudis, etc. have backfired badly, whereas support for modern european style democracies such as Israel has always been reciprocated, even if we have issues with their foreign policy at times?

When the oppressed become the oppressor, it's time for re-evaluation