The fact that Israel was created specifically as a national refuge for a historically marginalized and genocided people makes it very difficult to make nuanced criticism of that country. The rhetorical distance between anti-Zionism[0] and anti-Semitism is actually not that high. You can make a nuanced critique of Israel, but most people doing so would much rather consider the people and the government one and the same... and call for death to both of them. Conversely, early on in Israel's history, a lot of well-meaning people really did think we actually needed a Jewish-majority nation to prevent another Holocaust; so they had even more support than they do today.

Geopolitically, Israel made the diplomatic masterstroke of allying itself heavily with America, which makes it rather difficult to actually punish them through the official channels for such things. Even things like symbolic non-binding UN condemnations of the most heinous things Israel has done would get regularly vetoed by the US up until recently.

Furthermore, there's a question that most anti-Zionism doesn't have a good answer to: how do we keep another Holocaust from happening? Winding down the underlying Israel-Palestine conflict in a one-state solution means significantly diluting the Jewish majority in Israel with people who are, justifiably, very angry about the shit Israel did. Actually, they're unjustifiably angry, too; and one of the prevailing political parties in Palestine has made "commit another Holocaust" one of their explicit goals[1].

Conditioning any peace solution on special protected status for Jews might fix this, but the only difference between that and "special protected status for whites in South Africa" is an increasingly flimsy social context that's doing an awful lot of work. It would be difficult to balance the need to prevent another Holocaust against the needs of Palestinians, and the sort of right-wingers that like the idea of race wars would absolutely insist on their needs overriding the "other side's"[2].

Nothing I said above should be taken as justifying Israel's nonsense, or the nonsense of Hamas, of course. But Zionism relies upon all other alternatives either being unworkable, or worse. Israel is very much akin to an apartheid state, but all the countries that can really put pressure on Israel to stop that have no moral authority to do so. The UK and France are responsible for partitioning Palestine in the first place; Germany killed six million Jews; and America refused to admit refugees from that genocide. Zionism is banking on history repeating[3]; or at least people believing that it will repeat should Israel step back from the brink of ethnostate nationalism.

[0] For the purpose of this comment I will restrict "Zionism" to just the demand for a non-secular Jewish state in the land currently occupied by Israel and/or Palestine; i.e. Zionism in it's current form.

[1] https://en.wikipedia.org/wiki/Hamas_Covenant

Granted, this is #NotAllOfHamas, but party hardliners still insist it's their manifesto.

[2] Insamuch as you can even neatly divide the people in question into two "sides". In fact, Israel and Palestine do not divide neatly into Jewish and Muslim; there are plenty of Israeli Arabs and Palestinian Jews out there.

[3] When they aren't insisting that this is an "ancient conflict" to make their side seem more legitimate.