There's a lot of examples being given but no reasons...
Well, someone did say, SH is the sister site of NovelUpdates, so there's a lot of crossover with people who read translated works. So anime style genres and stories should be much more successful- but then, mecha isn't very popular is it?
Well, for that one it's because Mecha is a very visual genre. You don't watch mecha stories for the plot. You watch them for the massive robots bashing each other and getting torn limb from limb- in a family friendly way. Ah, Sports is the same way, isn't it?
Horror, Tragedy and all those are unpopular because Web Novels are read for escapism rather than catharsis. At least, that's my reason for disliking them. Considering how many people
say they like those genres, I can only shrug it off as weird when nobody else reads them.
Mystery is unpopular, because half the audience for mysteries just skips to the end of the book. In web novels, there
is no end of the book. It's a perplexing problem, no? A good mystery requires careful set up and time investment to try and figure out what's going- and they already stopped reading, didn't they? No patience...
Josei is also somewhat rare. I'd wonder if Seinan is rare too, but I think some people just treat it as a fancy word for the Adult, Mature, Smut categories. Technically the two are about "Growing Up", which is already a bad start for people who are reading web novels instead of getting taxed like good little social cogs. But then, a majority of readers are male, and don't relate to the problems a Josei story presents- or actively reproach such stories as whiny or feminist.
That's all for now.