Jehovah's Witnesses have taken aspects of the bible and followed only them. They enslave people into a works-based religion, which the bible speaks negatively about, and forces them all to act and be the same. It is simply a cherry-picking cult. Sorry to any JWers who would read this, but it is. I have no ill will toward you.
The reason that the Christian community, as well as other Abrahamic religions (minus Islam of course) support the Jews is because within the bible, God tells us to. It's that simple. Despite them sinning and being stubborn toward accepting Christ, God still considers them his people and knows that eventually their stumbles will be corrected. It is for that reason we do our best to show them the truth and in the meantime, support them.