THe M45 is an ugly car marketed to a middle-to-older aged crowd, and really only has performance to set it apart from its competition - that alone isn't enough. They tried to advertise it as some sort of modern musclecar, but that just doesn't work for a Japanese Luxury automobile.
The Q45 doesn't really have much charisma either, and for the price people would rather own a Lexus, BMW, or Mercedes because you get the name recognition and whatever else the name promises (reliability and service for Lexus, performance and ultimate driving experience for BMW, prestige and luxury for Merc). The Q also markets to older buyers, who just aren't as concerned with the performance, which is the key advantage it offers over its competition.
It really just comes down to the fact that Infiniti has done a very poor job of making a name for themselves. People immediately have images conjured up in their minds when they hear Lexus or BMW, but Infiniti just doesn't ring with many people. I'd say the company needs to fire a good portion of its marketing staff.