There is a two part answer to your question. First, I think the major difference is Obama and Edwards are open models. The key distinction between MyGOP and McCainSpace is they are a closed/restrictive model. It really reflects the difference in philosophy about the Internet that has developed.
The Democrats see the upside of the Internet as a tool to gather people around a cause, and to some extent they underestimate the potential downside.
Republicans, on the other hand, tend to overestimate the potential dangers, and underestimate the potential benefit. Republicans have not yet seen a race where the Internet was credited with a win. They have seen some of their candidates lose because of the net, but they have had little success.
The other problem is the nature of Republican voters and Democrat voters. I've done polling on this and found clear evidence that Democrats and Republicans behave differently online.
Democrats are generally more receptive to social media than Republicans. Republicans tend to be much more concerned with individual pursuits. It's most often Republicans that are bowling alone.
This is clear when you look at the counter for candidate MySpace friends. It's not a surprise to me that the numbers put up by most of the Democrats dwarf the numbers for Republicans. It's not a function of the age of people on MySpace, it's a function of what we're doing online.
If you're not inclined to social networking to begin with, and the process to get involved is heavy-handed and requires you meet the "approval" of someone else, you're not likely to make the effort.

print
email
delicious
digg
technorati
A tipster passed along this
What about the others?
David, if the major failing of McCainSpace is that it's yet another social network that requires you to build yet another profile and meet yet another group of friends, all in the service of raising money for the candidate, does this mean that other sites that follow this model (like Obama's and Edwards') are also failures? If not, what sets them apart?