I don´t think the hate for Micchi is the reason for Ryugen´s perceived low number of pre-orders.
In all honestly, i have been following Gaim like everyone else and i don´t find the character as despicable as much of the fandom makes him out to be. He is like 1/12 of your average Kusaka Masato.
But regardless of that, even though he is one of the more fleshed out characters in the show, i can say that he has had the least ammount of action: i cannot think of a single action scene or important plot moment where he played a pivotal role. Most of the time he is either playing second banana to someone else or just not fighting.
So yeah, even though he is a main rider in the show, i am not surprised that there is little demand for him since he is not very prominent as a Rider.