I've no idea why individual people voted for Brexit (there certainly wasn't just one reason), but it was reasonable to suggest that May's Brexit deal was going to be better than a Johnson led one. Parliament had the chance to go for a "soft" Brexit but decided it would rather bring May down in...
If it was only the ERG voting against it, it would have passed through Parliament. Don't forget the part that opposition parties took in blocking everything, calling for second referendums etc resulting in May resigning, her deal not being agreed and Boris ending up with a stonking majority and...