Of course it was mainly Jewish intellectuals who were the first proponents of Marxism and Communism ( of course Marx was Jewish himself) in Russia.
Communism was predicated on ideas of International Socialism whereby the nation state would dissolve to be replaced with an international order in which all shared in the principles of Communism. As you'll be aware Jewish people had everything to gain by the creation of such a society - in which the accusations levellled at them ( of disloyalty etc), and the hundreds of years of other anti semitic tropes would also dissolve and Jewish people could take their place as equals, with full citizenship.
It didn't really work out that way, though, did it - and Stalin if he hadn't died first had been hatching a plan to deport the Jews to far flung reaches of Russia. The Jews were never to be forgiven, by the Left, for prioritising their Jewishness and their Jewish identity over a dedication to International Socialism. Zionism represented a rejection of the vision and was seen as a betrayal of communist principles.
It has been said that some Left wing Jewish people, such as Alexei Sayle - who was brought up by Communist parents in Liverpool ( they would holiday in the eastern bloc during the Cold War and were active in Communist politics) still prioritise their political commitment to International Socialism over their Jewish identity - and is why they tend to frame their Jewish identity in terms of being opposed to Zionism ( the 'good' Jews). It must be a tough challenge to reconcile - and often involves being opposed or hostile to the rest of the Jewish community - including friends and family members. most Jewish people are Zionists at heart - and the recent rise in anti semitic attacks in Britain and elsewhere reveal exactly why Zionism became a thing in the first place.
Why are Jewisih people living in countries other than Israel being attacked in the streets, and on university campuses, and having to have police outside of synagogues on a Saturday - if Israel has got nothing to do with Jewish people?
Why do left wing students have to declare that they are anti zionists to be acceptable in left wing circles?