Because it wasn't advertised properly.
I've been repping my guild all week because I was told by friends to select my guild for the beta, yet I’m in a different server than them today. Turns out that, repping is not the same as selecting your guild, found it out today, so gotta be interesting playing with strangers for a week, I don't mind it that much, just enough for it to be my second post in a forum I never visit.
Why are players getting bogus or incomplete information?
Because they are getting it second hand.
Why are they getting it second hand instead of from the official source?
Because the official source lacks presence.
But it was on Reddit, Twitter, and the forums!
Well, not everyone reads the forum, that's a known fact, I'm more a ticktoker that any other social network, and reddit is popular in English but not as important for other languages.
But it was in the launcher!
Yes, the launcher that has an autologin feature that make it easy for you to miss it.
You are being annoying!, how can ANet reach everyone if none are on the same media?
Well they all play Guild Wars 2, I believe the perfect channel is obvious.
ANet already announces some stuff with an in game splash screen or in game mail, adding a banner to the character selection screen would be another option, imagine a window stating “You didn't select a guild for WvW Beta, would you like to join XYZ or put up with randoms?”, none is infallible by itself, but every option adds up. Abstracting from the game is the worst they could do, as there is no warranty that any other media will reach all players.
I sincerely hope this same mistake is not repeated for the actual release of alliances or any other future beta that features forced participation.