My initial thought is that this was due to bad timing. The first big comic conventions were starting up just around the time that newspapers started seeing a decline. Most of the nationally popular cartoonists were retiring or dying, and while there were a few newer stand-outs (notably Charles Schulz) the shrinking space given to newspaper strips and the greater attention to television news wasn't conducive to comic strip cartoonists holding a place in the collective consciousness of the American people.
But that doesn't quite explain it, I don't think. Because even with the diminishing significance of comic strips, they still overshadowed the popularity and respectability of comic books. And the presence of webcomics at conventions also contradicts the notion of popularity being a reason behind things.
But the webcomic angle does make me think of why/how they work differently than newspaper strips. Newspaper strips, almost by definition, need to appeal to a very wide base. As such, it's very difficult (though not impossible) make a deep emotional connection with readers because you have to work in fairly broad (i.e. bland) terms. Webcomics, by contrast, tend to focus on smaller niche audiences. The work can be more personal and, while it won't connect with as many people, the ones it will connect with, it does at a much deeper level. You don't need five thousand passive readers to give fifty cents, you need 25 energized readers to give you $100.
That distinction is perhaps the main reason why you don't see many newspaper strip artists at comic conventions. The comic con experience is about celebrating and highlighting your passions. And come on, who can be passionate about strips like this...