I think Christianity is a fine religion for non-Jews, the best of all available alternatives, to my knowledge. The reason Jews don't attempt to convert others, and in fact discourage conversion, is that the essence of Judaism is choosing virtue of one's own free will. While we may disagree on specifics of doctrine, the essence of Christianity is very similar. So I guess what I am saying is that Christianity is essentially a vehicle for bringing the lessons of Judaism to non-Jews, which accords with its historical development.
That's an unfair characterization... Judaism is NOT "for Jews only." The reluctance to convert comes from centuries of forced conversion by others against the Jews, as well as the belief that if one wants to become Jewish, they will be welcomed, but the initiation needs to come from those who wish to convert.
There's a saying though, that not everyone was born to be a Jew, meaning that if you're going to be an observant, religious Jew, there are many obligations, and many people are simply not willing or able to perform those obligations. Including the vast majority of Jews.
Mark