As a sys admin who manages a site that got a C+ I would disagree with
you that its the rules that are keeping many websites from maximizing
there potential. I would attribute the "pathetic" showing to two
things:

Information Control: my office wants to control the message from
start to finish and often times that means limiting the message as
well. We have the normal links to legislation, state/district
resources, tour information etc (all graded in the Golden Mouse Report)
but we lack almost all of the new media tools that seem to push others
over the top. While podcasts, embedded video, and even relatively
updated blogs and calendars would be nice (and easy to do) getting that
information vetted through our office is a timely process that often
leads to a "canned feel" that leaves a lot to be desired - not to
mention getting the boss in front of camera or a microphone to record
those pieces of media is tough. If your boss is more media friendly
(more aren't then are) it might be slightly easier to get some of this
material but then note my second point below to see why much of that is
left out on websites. I find that many of my colleagues in other
offices find these same roadblocks when trying to improve they're own
web spaces and integrate new technologies - not all but many.

Not enough skills and/or time: Too many sys admins and communications
directors do not respect the internet or technology in general. While I
am the sys admin in my office I am also a Legislative Aide with a
sizable issue portfolio - which means my time is heavily split and
often skewed towards my legislative duties. I do have formal schooling
and expereience in IT but I have found that unless your on a committee
or in a leadership office having someone with any IT training is very
rare and many offices don't value it as they should. This is not the
case for all offices but there are too many LC/Sys admins who happen to
be the youngest person in the office and thus given the IT
responsibility.

Lastly and not to be considered as important as the other two reasons
there is not a sense of urgency. Since the turnover in congress is so
low many don't see the need to innovate when communicating with they're
constituency - since most are multi-term incumbants who consistently
win 60% of the vote they dont feel the need to be more open and
creative in using the web.

Its funny, politicians require constant, timely, and accurate
communication with the people. The web is the best communication tool
since the invention of the printing press yet many don't place enough
emphasis or respect it enough to take advantage.