Abrahamic religions (Christianity and Islam) aggressively convert others. Why doesn’t Judaism do so?
Never ask why before asking if. Historically, Jews have sometimes persecuted and forcibly converted infidels.
Adding to Oreste’s answer, there’s also the example of Himyar, a late antique kingdom in southwestern Arabia (parts of present day Saudi Arabia and Yemen). Some form of Judaism, perhaps a bit unorthodox, is known to have been gaining ground there since the 4th c., until eventually it was adopted by the ruling elite and became the state religion (5th/6th c.). That created complex dynamics between Himyar, the nearby Miaphysite Christian kingdom of Aksum and the great powers of the time: the Chalcedonian Christian Byzantine empire and the Zoroastrian Sasanian empire. It also brewed internal strife, as the Himyarites had Christians among themselves.
Reports about the persecution of Christians in Himyar begin to appear in our sources from the last quarter of the 5th c. An Ethiopic synaxarium tells the story of a Christian man named Azqir, who was condemned by a court of rabbis and martyred in his native city of Najran. In the aftermath of his death, some forty Christian clerics, monks and laymen were also put to death in the same city.
In the early 6th c., Marthad’ilan Yanuf was king in Himyar. The Aksumites exploited (and exaggerated?) the renewed reports about persecutions to wage a retributive campaign against Himyar, which was led by a Christian Himyarite. Yanuf was toppled, and a local Christian was installed as ruler. A few years later, though, a rebellion broke out under the leadership of a Jewish Himyarite named Yusuf As’ar Yath’ar, who proclaimed himself king and toppled the Aksum-backed regime.
Yusuf launched a pogrom against Christians, which he gleefully reported to an international meeting attended by Byzantines and Sasanians. His goal was to wipe out Christianity from his realm. Well known is the massacre of Najran (523). Yusuf besieged the city and gave its people a choice: convert to Judaism or die. The majority (?) chose death, while others did deny their faith. In 525, king Kaleb of Aksum personally led a campaign against Himyar. He defeated Yusuf, replaced him with another Christian, restored the churches he had destroyed or converted to synagogues and welcomed back to Christianity those who had been forcibly converted.
Of course, there are a couple of things that need to be taken into account.
Firstly, the persecution of Christians in Himyar happened, for the most part, in the midst of political complications. Yusuf and his people probably saw the local Christians as potential, or even actual, Aksumite agents. That, however, is also the case in almost every religious persecution and/or forcible conversion. Nothing happens out of the blue, disconnected from its material, historical context.
Secondly, it’s true that Jews have often been reluctant to accept even voluntary converts. Rabbi Hiyya the Great (2nd/3rd c.) reportedly said: “Do not have faith in a proselyte until twenty four generations have passed, because the inherent evil is still within him.” The same, however, can be said about early Muslims — for some time, conversion of Christians and Jews to Islam was prohibited.
Thirdly, attention is often drawn to the fact that Judaism is more than a religion — Jews are both a nation and a faith. Again, that’s hardly as unique as it seems to be. In the early modern Balkans, religion was often all that separated Romans (aka Greeks) and Turks. To convert to Islam was “to become a Turk” in a (proto)national sense of the word: you changed your community and identity, even if you kept your language or customs. Much blood has been spilled along those lines.
That doesn’t mean there is no difference between Judaism on the one hand and Christianity and Islam on the other. If anything, there are more than 4.2 billion Christians and Muslims, but only 15 million Jews. One can argue, however, that what separates those religions the most is not any inherent mentality but history itself. Between the 6th c. BC and the 20th c. AD, there hadn’t been a proper, long-standing Jewish state. Not only did that prevent forcible conversions, but it also made voluntary conversions much rarer and solidified a sense of entrenchment among the Jewish people.
