The problem with Israel being "a Jewish state" is that requires the suppression and occasionally ethnic cleansing of its non-Jewish residents in order to prevent them from ever achieving enough numbers or power to make it not Jewish.

It's a fundamental contradiction embedded from the Israeli declaration of independence (which commits to racial and religious equality) onwards: what does it mean for a state, rather than individuals, to be Jewish?

The underlying argument is that it's necessary for the continued survival and security of Jewish people that there be a Jewish state, and unfortunately the continuous attacks on Israel suggest they might be right.