Why did native Americans accept reservations?

First, who is telling you that Indians "accepted" reservations? That sounds like something that fancy restaurants do. But not people. They did not accept being put on barren land with no game and no-good water. They fought it and the anti-Indian campaigns went on until 1890. That means that from the beginnings of an official policy of extermination, the Indians fought tooth and claw for 25 years. Their "acceptance" was what other people call "surrender or die."