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An increase in population density, which increased the number of idiots and tryhards.
(Also increased the number of competent users but not as much.)
Every release getting online multiplayer mode/focus: No single game gets enough attention, the set of "online video gamers" is too widespread among too many titles, not enough users to build a solid community in any of them.
Throwaway nature of the modern industry: Nobody plays a game long enough anymore. And server costs on the part of the publisher leads them to kill off the multiplayer in games more than 5 or 6 years old anyway.