Apollo's marketing game has been ridiculously on point. They are the only GraphQL client in the ecosystem that is fully compatible with any GraphQL compliant server [0]. And since it is pretty good, whenever one has to talk/market/propose anything about GraphQL in the community, they end up cross-marketing for Apollo.

[0] Relay has its own spec and can be used only if the server supports it. Other GraphQL clients are either not well maintained or do not support as many features as Apollo does (caching, subscriptions are two features that come to mind)