I don't think it is so much that the competitors are weak; it is that to google is to search. Humans are creatures of habit, generally speaking, so they won't switch to something else if they're satisfied with what they have. G is easy, convenient, and works well enough to satisfy most of the search-using population.
That said, there is not much room for anyone to significantly grow their market share (unless one of the others' businesses collapse). MS may pick up some share with Vista.