Because it's chock full of more personal detailed/focus than "every dating sim visual novel". Every choice really does matters in this one, and the LI remembers/is affected by every response; something that's very hard to do for one LI, let alone 5+.Why would it be difficult to have multiple love interests in the same game? That's every dating sim visual novel.
I don't know about you, but no other VN has met the choice quality standards of OL1 provided with Cove, and for good reason (I can only imagine the nightmare of catering every single choice to every LI, then making that choice affect all the other LIs you don't choose). They're stretching their limits with two LIs as it is...
Not to say other VNs suck or anything, it's just that you don't get that sweet, sweet personalized attention like OL does from your chosen LI when a developer throw in so many LIs; time, attention, and dedication is needed to flesh out romance routes (also fit them into the story, etc.), and the more LIs, the harder that job is = the longer/more work it'd take to finish. Which no one wants.
This is mostly the reason why anyone make single LI games, to get the most out of a LI as possible!