Well how many variations of the "heavy armor guy" are there, really?
Medium armor is very versatile - it's right in the middle. You can think of swashbucklers, asassins, rangers, druids, and shamans right off the bat.
And magic users? It's magic for Pete's sake, you can make up anything.
As for Heavy armor, I think Dartakc is right on the money when he said there isn't much more than the warrior and the paladin.
Really, though, I think it comes done to what's been mentioned before -- the GW franchise just doesn't have a lot of heavies about.
That said, it's also possible they may have an idea for a third heavy class, but it is too far behind in development to be released with vanilla GW2. Given all the changes that Arenanet is making to the traditional MMO structure, it wouldn't be too surprising if they came up with a new take on the heavy armor user. And it also wouldn't be too surprising that they'd want to wait before releasing it in an already unconventional MMO.
Edited, Nov 24th 2010 2:03am by apostolicalcoholic