Judaism wasn't (and in most forms still isn't) a religion focused on conversion or proselytization. The worship of the Abrahamic God began as a henotheistic religion focused on a God specifically the patron of the Hebrew tribes. While Judaism became monotheistic later, It's focus was still on the Hebrews as a chosen people born into a special relationship with the one God. You can see why they wouldn't be too concerned with spreading their religion. Besides, they were often more concerned with protecting their cultural/religious identity under repeated periods of being ruled by gentile nations (Babylon, Persia, Greece, Rome, etc.) Under Greek rule, for example, there was a policy of "Hellenization" which sought to spread Greek culture to their conqured territories. Some Jews saw this as a threat to their cultural identity as a chosen people, and in an uprising called the Maccabean Revolt in 167 to 160 B.C.E., pushed back against this cultural invasion. (The story of Hanukkah happens during this revolution).

What is more, the word "Jew" did not originally refer to a religious adherent to the beliefs and laws of what we now call "Judaism"; instead it was a political demonym for citizens of the kingdom of Judea. This political identity was central, since the Jews belived their God literally physically lived among them in the Temple in Jerusalem. When Judea was conqured and the First Temple destroyed by the Babylonian Empire in 586 B.C.E., Babylon began a program of deporting Jews. Jews call this the Babylonian Exile. During exile, stripped of their political identity as a people and the physical presence of their God, THIS is when "Jew" began to refer to a religious identity rather than a political one.

Now, there were certainly gentile converts to Judaism, but the focus remained on ethnic background, and these converts had to conform to this cultural identity.

It is impossible to understand the development of Judaism without understanding the development of Hebrew cultural (and national) identity. It is this ethnic centrality that makes Judaism a religion not focused on spreading. (Also, monotheism was a pretty radical idea at the heights of Greece and Rome, and most people general thought of Jews as cultish and unruly.)

(I have a BA in Religious Studies with emphasis on first and second temple Judaism.)