Because, for a casual, and I mean casual player, a Hunter is an extremely easy, straight forward, and convenient class.
Hunters have constant +runspeed, good enough damage even if not built correctly, ports to nearly every corner of the entire game, and like every other class, can easily faceroll 98% of landscape, even without a hardcore or raid-level player behind them.
That is why a Hunter is the most popular class.
Take them into actual instances, raids, some skirms or the Moors, however...and all of the flaws of a Hunter start appearing like cracks on an old sidewalk.
My other Warhorse is a Fleet Tactical Escort Retrofit,[I] Defiant-Class[/I], [I]U.S.S. Say My Name[/I], NCC-93144