I think Marat is so popular because he is different from all other players, he is a very interesting character, he does some things that are unique and he has so many different qualities... his personality is one of the extremes, he is so f*cking funny and a bit crazy in a very natural way, always entertaining in matches, has a firy temperament on the court, is so unpredictable, says what he thinks, his easy going attitude off the court, the cool side, yet he is a very nice and sweet guy with a big heart, (very popular with fellow players, fans and press) his charming side, his sensitive side, he is very smart and wise and and and...
and of course he has one hell of a game, and when its on its one of the best or maybe the best, but unfortunaly mostly it isnt. Anyway, i think another reason is, that he has such big mental issues that he once plays amazing and the other day he wouldn`t win against his grandma. He had so many ups and downs in his career and thats interesting for the people to follow him. You never know whats gonna happen. It all depends on the way he feels each day.
Another factor is that he is very good looking and expresses an unbelievable sexyness (without trying), that attracts many women.