Let's not be too unmindful of the probably many who choose the Danish or German for looks, RP or other reasons and not just stats. I have personally always preferred speed and always used the Two-handed Sword for duels in Native (and actually found the GS to be a bit cartoonish, too mainstream and anachronistic in comparison to other WB weaponry), where as good as everyone uses the GS in competitions, but in cRPG I can use a sword made by my forefathers and which I like the appearance of, and which also fits the timeframe of other armaments in the module. Others, many I guess, simply like to choose the biggest and most hard-hitting sword they can find, and many are accustomed to Zweihanders from pop-culture, which I think is perhaps the main reason for the prevalence of these swords on the servers.
And yes, as Tor touches upon, many have probably also been inspired by other good 2handers, especially in the early cRPG where there were many old duellists from Native using GS (and 2h lolstabbing was pretty ridiculous back then) with a lot of fighting experience in a time where the majority of the cRPG population had only just stumbled upon cRPG/WB.
That said, and though the weapon is expensive it definitely has great stats and I wouldn't mind them being slightly adjusted if other weapons were balanced accordingly, like the poleaxes which I personally find to be the strongest, all-round infantry choice in cRPG. I just don't think it will decrease their number as much as we may like.
