First mover advantage plus the effect that having the majority of users can improve the quality of their results (in fact Google was *not* a first mover in the search space, but now they are entrenched). In the internet, code is law, and Google has a good amount of defining many technologies in widespread use - and more importantly, the way to learn about them.
A few weeks ago I read an analysis by a Mozilla blogger (which I can no longer find) of how, now that pagerank is less and less useful due to link farms and spammers otherwise attacking their algorithm, search quality depends largely on analysis of search terms introduced by users and the results they find interesting. This is a chicken-and-egg situation for any competitor: you can only improve your results by having more users, but they won't come if your results are not better than the market leader's. This is a natural monopoly, but one created by network effects and thus of the kind that can only be displaced by a disruptive process, not by regular competition.
And there's a similar effect for advertising - if there's a natural monopoly over the space were all users reside, then you must advertise in that platform in order to have enough eyeballs. It's the same mechanism that produced a lock-in for Facebook and Microsoft platforms back in the day - you go there not because the product is better, but because you need to interact with everyone who is doing the same.