Part of the reason Blue (and Brainstorm) is so dominant is because many of the best players recognize that Blue gives them the most opportunities to use their skills. Many non-Blue decks tend to be very draw-dependent. Pros play Blue because they want to give themselves the option to outplay their opponents. There are a lot of balance issues why Brainstorm is cornerstone in the format that I won't get into here. However, just on a practical level, the reason Blue is such a presence isn't because Blue is that overbearingly overpowered, but because Blue gives good players the opportunity to play to the best of their abilities.