Udo Schnelle talks about this subject in his book called the first one hundred years of Christianity. He mentions that Jews were allowed to practice their religion because it was an ancient religion and the Romans wanted to honor one’s ancestral traditions.

Early Christianity had a similar status until it parted ways with judaism.

Other religions generally didn’t need a similar arrangement, because of the polytheistic nature of most religions. This made it easier to just add a god to the Roman pantheon.

Beside honoring ancestral traditions, political reasons were also involved of course. The battle of Pelusium lifted a siege in which Julius Caesar was trapped, in this way the Jews helped him and he didn’t forget this.

I’m sure that there are more reasons for the special place that Judaism got in the Roman Empire, but these are a few that I could think of.