To Love Ru! I want to buy the manga in English so bad. I don't understand how it hasn't been licensed in America. T-T
It's because it's too long. Long series don't usually do well up here, because the sales slowly decrease after each volume release.
The first series is 18 volumes long and Darkness is about another 10 volumes and still ongoing.
It's the same with Freezing, which is 24 volumes and still going strong.
Though, obviously, there are exceptions. Viz has published titles that are way longer.
Mostly of the shonen genre. Some of which are well over 60 volumes by now.
Yen Press also seems willing to take chances on longer running series. And they're always willing to take licensing suggestions.
Kodansha has Fairy Tail which is pretty long as well. Though that seems to be an exception with them more than anything.
So, it's more a combination of genre and length. Although, considering how well Monster Musume is selling, that could change.