Because none of the disciplines mentioned above can, on its own, offer sufficient expertise to address the complex design challenges presented by new media. To answer those questions, we need a multi-disciplinary melting pot to bring together all the appropriate principles and techniques from established disciplines and put them together into a meaningful whole. The role of Webmaster has been incredibly important for the commercial Web's development. Webmasters took responsibility for making a suite of new and unknown technologies work in practical ways for their employers. In doing so, Webmasters provided their organizations' Web education.
Most sites are too large and complex for a generalist to manage.
There are too many skills, as I've mentioned, for one person to provide in any reasonable fashion. That's why you don't see many people going by that label any longer. Webmasters have been replaced by hordes of specialists, such as graphic designers, information architects, programmers and editors, not too mention project managers who tie their teams' efforts together. Let's face it, the best sites out there today are far better than the cream of 1995's crop, or perhaps even last year's. Many of us just don't get too excited by the Web anymore.