Popularity
Why is Windows popular?
Because Windows has been mass produced. Same reason applies to Android's popularity, and to Google Search's, because the latter was the default on your web browser and on Android too. Reversely, why is desktop GNU/Linux not popular? Because it has not been mass produced. By mass production it means a particular type of distribution which is mostly successful as a means to popularity.
Why popularity do centralization?
Because a popular thing attracts many people to it and this forms a centralization. There is nothing wrong with being centralized as nothing wrong with being popular. Decentralization efforts will eventually be centralized whenever one point in the network being more popular than the others. By resources, to be fair centralization is lighter and easier to implement while decentralization is not more implementable and not more sustainable.