Any realistic business model for webcomics has to consider genre differences.
I think t-shirt sales will continue to be the anchor of most successful webcomic businesses. However, it's widely known that some genres have less success vending t-shirts. Fantasy and science fiction in particular tend to report disappointing results.
Since I became aware of the issue, I've been thinking about possible causes, and talking to people who work in those genres. Below are some ideas for audience consideration.
Understanding the problem is the first step toward making a future webcomic business model that is dynamic enough to recognize and account for genre differences. Webcomics derive richness from their diversity and it benefits all creators if they aren't crowded into a narrow creative zone.
It's also important to know about the problem before starting a webcomic-based business. This is a shortcoming of the one-size-fits-all business models that are floating around.
Here are some possible reasons why even the best fantasy and science fiction titles often face challenges selling t-shirts:
The emotions don't translate well to t-shirts
Drama, suspense, heroism and earnest quests make great epics but poor snapshots.
The art style doesn't translate well to t-shirts
These genres are often more realistic and less cartoony than others. That's not t-shirt art, which is much more based in graphic design.
Stock characters that translate well have been done
How many shirts with dragons are out there, for example?
Stock characters that translate well are incomplete without a message
Dragons and griffins have been used since the days of heraldry for the combination of animal wildness and mystic charm, and still carry that message in some renderings. Unicorns imply innocence. Dwarves with battle axes scream, "D&D Geek." Even D&D Geeks don't wear them.
These messages are clunky and don't appeal to buyers seeking a more individualized message.
They lack humor
Webcomic T-shirt design thrives on wit. Though humor comics exist in both fantasy and science fiction, the seriousness of the quest often squelches opportunities for laughs.*
People with experience in t-shirts will usually cite this as the most important reason for sales differences.
They have a smaller fan base
I suspect many people who read fantasy comics also read humor comics, but I don't think the reverse is as true. When we measure traffic to a comic for purposes of estimating its potential t-shirt market, humor comics benefit from "easy in, easy out:" they can pick up and lose hundreds of readers a month without losing market size. In fact, this may increase it, as it means more total people reading the comic and being exposed to t-shirts for purchase.
They have a smaller talent pool
People tend to create comics in the genres they themselves enjoy, but if a genre is less popular, there are fewer creators able to create break-out comics. Few people would have predicted the incredible success of Harry Potter among otherwise orthodox readers. People who attract interest to a genre without becoming hacks and sell-outs are rare and they can influence perceptions of what looks good on a t-shirt.
* Today I was shown some designs popular on the NASCAR and wrestling circuits, and they had emotional messages based on defiance, menace and threat. One senses that even drenched with machismo, a witty message would have to be very watered down to appeal in that crowd. Just as the emotional message varies among audiences, it varies among comics, and individual titles must adjust their expectations accordingly.