In addition to Max's point about how Minisiege uses the "standard" melee speed used across NW, I will add that increasing the melee speed only makes the game more fun for those players who are already more skilled, and makes the game less fun for anyone who is less skilled or who has a higher ping. The faster speed would only accentuate the differences in skill and ping that already exist.
Experienced players may have the slight annoyance of things moving a bit slower than they'd prefer, but inexperienced players will actually have a chance in combat. As a result a larger number of people can contribute meaningfully to their team's performance and thus find the server fun. Likewise any of the substantial number of people from outside Europe who play on the server would be negatively affected by an increase in melee speed, regardless of their skill level.
Minisiege attracts a large number of players of all different skill levels and from all over the world. Increasing the speed would take the 5-10 players that get the most kills per team and give them even more kills per round, but only frustrate everyone else. A few select players would effectively become gods in the game, and everyone else will go find other servers where they can actually play.
If you want faster melee speeds, there are groupfighting servers and other places where people are closer in skill and geography and where melee is the only focus. However Minisiege focuses on the wider concepts of goals and teamwork, neither of which is served by increasing the melee speed if that increase makes the game arbitrarily more difficult for most players.
For some added context, the Official NA_1 server does not use faster melee speeds for precisely the same reasons.