I think the issue with the way you've posed the question is that you're trying to define Judaism in vacuum and messianic Judaism is just another form of christianity. Early christianity was a splinter group of (normative) judaism and it stayed small it would still be considered one. For example, the karaite jews made a dramatic break with rabbinic Judaism, but I think this sub would be more sympathetic to them in labelling them Jewish as opposed to jewishly-oriented christians.

So I think it really boils down to history and culture. The early christians made an irrevocable break from the Jewish people and you can't rewrite history and say christianity isn't its own religion or that its defining principle is the divinity of Jesus. In contrast humanistic/liberal Judaism still maintains a connection to the Jewish civilization/people/religion. Also I think shedding beliefs or being lax in observance is a much smaller break from historical practice then adding on beliefs of outside religions.