You ever heard of Jews for Jesus?
Not every person born to a Jewish mother remains a part of the Jewish religion, nor do all Jews dismiss the Christian theology as a 'false prophet' situation.
Nah, the shit on reservations is way, way more complicated than that, and they don't get a lot of 'handouts' due to the way Federal vs State vs Tribal jurisdiction/authority extends.
Also, the Fed gov involuntarily sterilized a lot of native women back in the day, and usually reservations are on the crappiest or just least useful (at the time) piece of land the Feds were willing to give up.
Add in some old tribal grudges and bad blood from before any of them even saw a white person, and some things are issues the white man did not start; seriously, look up how the Apache and Comanche were viewed by their neighbors, or the pueblo vs mesa cultural divides.
Then there is the issue of the Bureau of Indian Affairs possibly being the most useless and corrupt agency in the entire Federal government, and things just keep getting worse for most reservations.
once you convert to another religon thats your religion.