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Because it was the new big thing back then, and any kind of online services will fall some day anyway, be it through the death of the servers or the decline of users.
It's worse with BW(2), though, both of which lock away a ton of functionality through Dream World and other online connectivity (wifi mode battle subway, medals in BW2, Join Avenue, etc.), which is now gone.
You can *technically* revive it, but good luck getting players.