While intelligence and strategic thinking are very much a factor in the making of a Chess GrandMaster, the key is the number of games played and scenarios seen. This is because chess players don't actually evaluate every possible future move. It has been found that a Grand Master does not evaluate any more moves than a sub-Grand Master. They just evaluate better ones because their brain can recognize more patterns and discard those that do not need to be evaluated faster.
For more info, read about chunking theory and Chess.
Encyclopedia of Educational Technology Chunking (psychology) - Wikipedia, the free encyclopedia