He wasn’t sure why anyone in The Order wanted anything to do with him anyway? To them, he was a death eater. He was dangerous. It didn’t matter the good that he did. It wouldn’t matter that he nearly died trying to stop Lord Snakeface. So what if Regulus wrecked his body trying to stop Voldemort? None of it would make any difference to anyone in this room (except you). In The Order’s eyes, Regulus would always be "tainted."