Tim Bale (The Conservative Party: from Thatcher to Cameron) discusses this and gives some good sources in the notes.

Essentially, the MPs ousted Thatcher because they thought she would lose the next election. The party was divided over Europe and unpopular because of the poll tax. Heseltine was popular in the country but represented an ideological shift from Thatcher back to the centre. He was also passionately pro European and would have inspired a backlash from the Eurosceptic wing of the party. There was also significant personal opposition to him from Thatcherites who could not forgive him for ousting her.

Major was less well known but this meant he could be portrayed as a softer Thatcherite. He also had Thatcher's personal support. One of Bale's sources said 'We knew he wasn't a hard core Thatcherite but he was the best we had.'

While Thatcher got 204 votes out of 372 this wounded her fatally. It still meant there was a Thatcherite base more than big enough to elect a leader. Major actually only got 185. Heseltine's 152 against Thatcher was an amazing result but it was his high water mark. It is hard to imagine someone who voted for Thatcher switching to Heseltine whereas 21 anti Thatcher voters did desert him once she was disposed of.

Michael Heseltine was probably too centrist to lead what by 1991 was an increasingly right-wing and anti-EU Conservative party. Major would himself find his prime ministry ruined by the so-called "bastards".

Beyond that, there was a narrative doing the rounds at the time (however true) that the Tories always valued loyalty above all else and that he who wielded the knife seldom got to sit on the throne. By 1991 Margaret Thatcher had completely lost her touch politically and the Tories would have been obliterated had they retained her as leader, but they still didn't want to shoulder responsibility for actually dumping their great icon.