One possible reason for the relative lack of Samaritans in the world is the place-based nature of the religion itself.

The Samaritans were physically dispersed by many of the same forces that led to the Jewish diaspora, but the religion was not as often spread or maintained in their new communities. Part of this was their unwavering focus on their temple, which remained for centuries after the Second Temple of Judaism was destroyed. "These factors explain why the Samaritan Diaspora was never able to develop in the same way as the Jewish Diaspora. One fundamental factor was that the Samaritan religious centre, the temple at Gerizim, was in existence until at least 484 A.D. and served as a spiritual and cultural centre which could not be rivalled anywhere in the Diaspora." (https://biblicalarchaeology.org.uk/pdf/ajba/02-3_107.pdf)

It's worth considering that the version of Judaism that survived was not the only version: much of Judaism in the era of the Hebrew Bible focused on the spiritual life at the Second Temple. The version that still exists today was deeply influenced by the Pharisees, who "asserted that God could and should be worshipped even away from the Temple and outside Jerusalem" (https://www.britannica.com/topic/Pharisee)

Judaism underwent a transition to a religion that could be practiced anywhere. Samaritan religion does not seem to have changed in this way, and didn't spread as widely.