I just find it odd that having a person of the Jewish faith was so important to her, she would have walked away. Yet married someone who took the faith simply to marry her. Not because they wanted to.
It's possible that he had some vague believe beforehand. Going from strict atheism like the OP to a genuine conversion seems unlikely to me, yes. But from some sort of monotheistic position? More plausible imo.
And we do not know why he ultimately took the faith. Converting to Judaism (particularly orthodox but also reform and masorti) is a lot of work. You need to study the religion, usually also the Hebrew language, immerse yourself in the community of your Rabi, go before a court (the court may very well say no), undergo circumcision (or spill a ritualistic drop of blood if already circumcised) and step into the mikveh.
It's a lot of work and also costs money. It's imo therefore a fairly convincing show of some sort of commitment.
As for the why?
I obviously do not know what she thought.
But conversion is ultimately also a promise to hold up the commandments, isn't it?
Sure, believing is one of them. But there are others. 🤷🏻♀️
And I would guess that it ultimately is about the education of the children and family life for most believers... Or simply about appeasing their family which seems much more problematic to me.
Anyhow, think it's perfectly fine to not marry somebody for whatever reason.
As long as that is communicated clearly. Definitely before moving in. And no, communicating some sort of preference isn't the same as making it clear that it is a real, hard limit.
And it seems to me as if it the OP is really incompatible with her OH...
Knowing when to cut your losses is a skill as well.