Why do you think that GW hasn't made new editions of the game? I'm talking about new miniatures, rules, and more...
The other question to ask is what rules have you access up to Ale? If you've not had access to Fanatic Magazines, track them down via ebay so as you have access to a wealth of other material. Tom's Boring Mordheim Forum has a whole heap of stuff that Da Bank here has linked to so contact him also.
That said, it gets down to the financial side of things and the fact that GW's a company needs to survive. As such they invest into products that will sell more mini's and it has been stated repeatedly that the company aims to sell miniatures over developing rules further et al. As Bank says, Mordheim, due to its fantasy setting, in its own way has new models released every month as WHFB is the overall setting the game is based in and all the models and rules come from that setting. Alo of the collectors range of miniatures whilst not specifically Mordheim aren't specifically WHFB either but suit use in Mordheim better.
Rules wise, a variety of sources online independent of GW develop expansions for the game but as a whole, the game works relatively well as it stands, a few FAQ issues aside.
Cheers,
Ram.