Its not a question of 'being' supported ... Begining 1990, Liverpool have had modest success compared to United.
Yet they have or until recently had a huge fan base, comparable to United. The issue is that Liverpool's fan base is an ageing one, while City and Chelsea have a lot of younger fans coming in. Ronaldo played a significant part in converting a lot of young fans (especially outside England) into United supporters.
As an example look at me ... I only got interested in United (not a fan), Busby, Munich, Docherty's affair, Ferguson kicking out the drinkers, etc. after Ronaldo joined Madrid. I am marketing gold for a sponsor since my recall (and association) on United is better than most casual fans ... Ronaldo did that to me.
EDIT: I have it on strong authority that advertising companies also look at Fan Forums to gauge Fan interaction/stickiness/involvement-ness .