Judaism wasn’t an ancient religion constantly nibbled by agents of the neighboring blasphemous partisans and lazy locals who wanted to, you know, go dance around the asherah all the time.
Judaism was one of those blasphemous everything-goes Canaanite religions to begin with, and the supposedly ancient form rose out of that; not through it. It wasn’t an immutable thing handed down from Adam to Abraham and so on; no, Judaism of old was just another Canaanite folk religion, and the kind we usually call that is a much later priestly invention, from seven or so centuries BCE. The continuous shrill ranting against the dumb folk worshipping wrongly was not directed at new perversions… but at the old orthodoxy.
Christianity (in the sense of the major, long-lasting general kind) wasn’t a clear comprehensive original teaching from Jesus himself, plagued by constantly twisting and wilfully lying heresiarchs.
No, what came from Jesus was a vague and small thing. There was a hundred ways of interpreting it, and all were used; a million fractal details that were missing, and they were all proposed every which way, with none of the proposals intrinsically “heretical” or wrong, but those that didn’t win very much retroactively so. And the scribes weren’t all that able or willing to be rigorous about who had said what — a lot of theology is just guesses and coin-tosses justified with really tortured readings of single verses, and the layers above that are much more later inventions and fan fictions and nothing more. Many things did not became heresies for centuries because it took time for the crazy mad differences to be settled so people could guess about and schism over the small ones. Something as small as the Catholic-Arian difference — was the Son created by the Father? — just couldn’t be feuded over before the sects agreed about most other things. (“What do you mean created? The Demiurge and Jesus-God are not in any way related!”)