Judaism is one of the oldest religions, but there aren't many Jews compared to Muslims and Christians.
What are main reasons behind that?
There are many reasons why Jews have a low population. Depending on your point of view, different reasons will be "more true" than the others.
TofeeqAhmad, I appreciate your changes to the question. The answer, however, remains that mass murder and an aversion to proselytizing has prevented much growth. At our very founding, according our tradition and the text of the Torah, 70 people went down to Egypt as a family/tribe and developed into a small nation within a nation. We grew by leaps and bounds. And then, during the Exodus, only 1/5 came out unscathed. Then, at Mt. Sinai, due to the sin of the Golden Calf (mere months after leaving Egypt), thousands more died. Over the course of the journey through the wilderness, we lost some individuals to war and more were lost to plague.
In the land, there were periods of relative tranquility, during which times there was some growth, but there were also many, many periods of war and exile, during which times the population fell, sometimes dramatically.
Later periods had ups and downs, but once Christianity developed into a full-fledged religion with an empire behind it, there was very little the Jews could do to grow their numbers. Even sustaining their numbers became a challenge. From pogroms to inquisitions (the most famous was the Spanish Inquisition at the tail end of the 15th century, but there were many, and they occurred in many places, including places now friendly to Jews, like Great Britain) to outright genocide, there really hasn't been a time when the Jews have had unlimited opportunity for growth.
Add to that the fact that, for most of our history anyway, we have not really outwardly proselytized, either by force or by reason, so our growth has always depended on our ability to naturally increase our numbers by our birthrate exceeding our deathrate. Since genocide, slavery, mass murder and torture significantly alter that ratio, any significant growth we've experienced has historically been offset by massive losses.
And I thank you for putting me into a very depressed frame of mind this morning... :(