I think part of it is when a comic is labeled romance, it's easy to know what to expect.
It doesn't mean that romance can't be as original or as diverse as other genres. But for the most part, when a creator labels their comic as romance, the fans seem to know what they're gonna like about that story?
I label my comic as Drama but that really doesn't tell you what you might like about my story. I also used to label it as Fantasy, but that also doesn't tell you what's good about the story -- it doesn't even tell you much about the setting!
Anyway, it's a theory. Feel free to add to it, refute it, etc. 