Originally Posted by
HappyBandit
not an expert on aerodynamics but I'd imagine it has to do with increased drag and entering increased stall speed with altitude.
I think you will find its more to do with compressibility. Flap and gear will have limiting mach numbers. Not sure on the specifics of the q400 but I think you will find that its Vlo will crossover to its max mach number for the gear at 15,000. The mach number is probably not stated though, hence the altitude restriction.