Because they have such a small share to begin with. Here's an example, completely making up numbers.
Say Apple has 3% share in a quarter where 100,000 total computers are sold (I know that's ridiculous, but this is an example.) So Apple sells 3,000 computers while everyone else together sells 97,000.
Say that Apple grows twice as fast as everyone else, 20% growth to 10% growth. In the following quarter, Apple will sell 3,600 computers (3,000 x 1.20), while everyone else together sells 106,700 computers (97,000 x 1.10). That means that Apple has only upped its market share to 3.26%.
So when you start out small, it takes a long time of outperforming everyone else by an awful lot to make any headway.