Short answer: many of the late 1800's/early 1900's Communist leaders were the intellectual elite. Think middle-class university lecturers, teachers, doctors, or the like. Many other Communists were highly educated as well, even if their jobs were more mundane (i.e. engineers) Their views were fairly liberal for the time, and Communism was something akin to humanism a century before.

Many of these people (referred to as Intelligentsia as a social class) were pro-Communism on ideological grounds.

Jews have always been very prominent in academia.

The Russian Empire was also openly anti-semitic (whether it was following the general public sentiment at the time, or actively against the Jews is a question for a different discussion), so many Jews harboured little goodwill against the Tsarist government. Coupled with their generally good education, many became revolutionaries that wanted to remove the Tsar from power for a more egalitarian form of government.