With tens of thousands of webcomics out there (The Webcomic List follows nearly 14,000 webcomics, though admittedly not all of those are currently active), there's bound to be something for almost everyone. Some genres are more prevalent than others (college dorm sitcoms seem particularly well-represented), but for the most part if you're looking for something particular then there's a good chance that someone else is already making it.
From what I've seen, print comics haven't been able to offer anywhere near this range of choice until fairly recently. The most likely reason is because comicbook stores have a finite amount of space in which to display their wares, and will only tend to stock comics which they can be confident will sell. Stocking comics which might not sell means that those comics either stay on the shelf and take up space (irksome for the store proprietor, as such unsold books are basically wasted money), or get remaindered and sent back to the distributor - in both cases wasting valuable shelf-space which could've been used to display a more popular (and thus more saleable) title.
This works to limit the range of comicbooks available to the reader, as the most popular genre elbows out the less-popular genres. In the English-speaking world, mainly served by the American comicbook industry, the medium has been dominated by the superhero genre for fifty, sixy, seventy years - superhero comics have been the most popular with the readers, so they're the ones that comicbook stores are most willing to stock; because they're more popular with the stores, comicbook publishers focus on that genre rather than other, less-popular genres.
The situation reinforces itself as more and more shelfspace is devoted to superhero comics, and the readers who aren't interested in superheroes stop visiting the store because they can't find their preferred genre amongst all the dudes in tights punching each other. Soon only the superhero fans are left, the other potential comic readers having abandoned the medium because of a dearth of their preferred subject matter.
That's not to say that other genres are totally unrepresented in American comics, but that they've certainly taken the sidelines compared to the superheroes, and the impression I get is that the American comicbook industry has suffered as a result - the superhero fans are a pretty small subset of the total potential market, but until recently that wider market has been almost entirely neglected.
It's as if the TV networks decided only to air shows from the Star Trek franchise - all the people who would prefer to watch soap operas, documentaries, sports, drama, murder mystery and sitcoms would just switch off and go find something else to do. That's no good for anyone involved in the industry, which is why it's so important to demonstrate to non-readers that comics have something more to offer.
In terms of print publications, European comics are a good bit more diverse than on the other side of the Atlantic, but it's Japanese manga that seems to be the closest print alternative to webcomics in terms of offering range of subject matter. This is perhaps why manga is finding such an enthusiastic audience in the West - after a half-century of dudes in tights, there's a sudden influx of other kinds of story that aren't steeped in the superhero tradition, which are thus more compelling to a wider audience. Mystery, romance, fantasy, high-school drama, to name just a few - if you can find a genre in the novel section, you can probably find it in the manga section too.
This is what mainstream American comics have been lacking for so long. It's a void that the small, versatile, independent content-creators of the webcomic community are in an ideal position to fill, and that is what makes webcomics such an exciting field in which to be involved.