The "alternate cryptocurrencies" sub-board has been around for a good deal of time, yes?
It's been around for one or two years, I don't know exactly.
Why couldn't other altcoins exist concurrent with bitcoin?
It's in the interest of the bitcoin community to only promote one blockchain based on bitcoin software. Promoting an endless number of blockchains with virtually identical software is the same as ignoring the 21 million coin limit.
To the extent that this happens, the appeal of all bitcoin-like-currency is reduced because the perceived scarcity of the coins diminishes, and I think this hurts growth in the total market cap.
It has the added disadvantage of fragmenting the network, making it less useful to each individual.