I thinks its just easier to write and sell.
The best selling and oftem the most expensive eroge are usually love themed which is why moege´s are oftem the "big boys". Love stories needs buildup, character development and interaction in order to make love flourish which is why they are usually more oftem than not witouth that much H-work.
Now with the cheapest eroges (Like the usual you find in DLSite) you find more rape/violent works because its so easy to write: rape needs no buildup since it only needs a rapist to want to rape and done, mindbreak dont need much else than lots of sex and making a character shift from not liking it to enduring it to enjoying it, NTR the protagonist and heroine are already in love so it just need the faceless bald old man to want to NTR the protag, monster rape just need the heroine being defeated by the monster and done.
Love is just the hardest to properly convey in order to sell it to a demographic and not to mention it cant really have that many fetishes like harcore ones can. And as they are usually cheaper they are more accessible and thus easier to sell.