One of the reasons Thatcher was challenged were polls showing the Conservatives ten points behind with her as leader and level with Labour if Heseltine were leader.
Backbenchers in marginal seats, facing electoral defeat and unemployment if Thatcher remained leader, came to the not unreasonable conclusion their job and career prospects would be better served with a new leader - the polls flushed out Heseltine (as did Howe's resignation). He was forced to challenge but he only succeeded in being the assassin - when new polls showed Major doing as well if not better than him, combined with Thatcher's endorsement of Major, allowed those backbenchers who had deserted Thatcher to save their consciences by supporting her successor.
Backbenchers in marginal seats, facing electoral defeat and unemployment if Thatcher remained leader, came to the not unreasonable conclusion their job and career prospects would be better served with a new leader - the polls flushed out Heseltine (as did Howe's resignation). He was forced to challenge but he only succeeded in being the assassin - when new polls showed Major doing as well if not better than him, combined with Thatcher's endorsement of Major, allowed those backbenchers who had deserted Thatcher to save their consciences by supporting her successor.