IMO For grown men to like this its a fear of adulthood and the responsibilities that come with it. Hence the regression to children's programming.
Now why a "girls" show is another question that I cant begin to fathom why grown men like that. Now I want to be delicate here.....but lets just say I think if your a grown men watching My Little Pony......it would be relatively safe to predict that their quality of life may not be as high as the average adult.