I think the reason Richard was more pro-active with Maria in Darius's route is because Darius came by the shelter in both his classic car (which Maria immediately knew the year and model of) and his custom motercycle (which Maria also loved) with the Main Character (MC). None of the other routes characters had anything special vehicle wise like that.
Darius also did other things to spoil her and I think Richard ended up feeling a bit threatened/jealous of how Maria was reacting to Darius. Hence Richard making that purchase (to surpirse Maria) and his opening up to her about how he feels.
Of couse, Richard was none too shy about his interest in Darius too, if the MC didn't go after him. I look forward to Richard's route coming out. I think it's really going to be something different.