Michael's primary reason for staying back to retrieve Rachel is that they love each other. While it is a secondary plot line, the romantic relationships between David/Rosalind and Michael/Rachel have been mentioned throughout the novel. While David and Rosalind's relationship is more in the forefront of the novel, there is a lead up to Michael and Rachel's:
Rachel's afraid. She's crying inside. She wants Michael.'
' Did she tell you that?' I asked.
Petra shook her head. 'No. It was a sort of behind-think, but I saw it.'
' We'd better not say anything about it,' I decided. ' It's not our business. A person's behind-thinks aren't really meant for other people, so we must just pretend not to have noticed them. (Chap 16)
It could also be argued that Michael understand he and Rachel have a special bond because of their telepathic powers and the experience of the group. As we saw Michael explain in chapter 10:
Michael told her it'd be like pretending to have only one arm because the person one wants to marry has only one arm. It wouldn't be any good -- and you couldn't keep it up, either.'
Michael understand Rachel, loves her, and doesn't want her to suffer alone.
She's quite alone,' said Michael. 'Would you leave David alone there, or would David leave you?'
There was no answer to that. (Chap 17)
I would assume that the reason Michael chose to go back for Rachel is because he loves her. He compares his relationship to Rachel with that between David and Rosalind whom we know are in love.
" 'She is quite alone,'said Michael. 'Would you leave David alone there or would David leave you?' "
He goes on to suggest marriage would happen betwen Rachel and himself,
"... waiting for the day when we, or perhaps our children, would be found out..."