If Jesus was an unquestionable manifestation of God, why only few thousand Jews became Christian?

The answer is precisely what you suggest: the evidence that the Jews of that day and age saw was unconvincing. Let's underline that: The Jews who who were there to see Jesus and hear the teachings and witness the alleged miracles and all, *overwhelmingly* rejected him, did *not* think he was the messiah, did *not* think he was in any way divine or special, did not even consider him a valid prophet. They were not at all persuaded. Heck, there was more Jewish support for other false messiahs, like Bar Kochba; by comparison, Jesus comes in distant last place among the Jews. This is why the early Christians were forced to seek converts from the pagans, because the Jews weren't buying it. And this is why they decided to throw out The Law (despite the fact that Jesus *upheld* and *confirmed* The Law), because they couldn't get pagans to buy in if they had to be circumcised or keep kosher or any of that. And thus, what started out as Christianity became Paulianity, and that is why it has so widely veered from Judaism, and how you get this ridiculous contradiction of a bunch of people proudly proclaiming they are NOT Jews (and therefore don't have to follow Jewish law) yet worship *a Jew*.