M5's are still as stiff as an Ultrasonic pad which is still in the "too stiff" category for a lot of consumers. Couple that in with the Vapour gloves both being more popular than the Supreme versions at retail and people wanting a matching set, it's no big surprise that Vapour still comes out on top.
This is my own thoughts, so not really based on any tangibles, but, I have this feeling that users looking for a stiffer pad in general are more likely to get a Pro level pad as they start to trend into the less "casual" side of the playing sphere (if that makes sense). The only guys I've seen rocking Mach or Ultrasonic gear at the multiple leagues around here have always been "better" goalies. Same with Px9 gear now that I think about it.
Not saying that owning Ultrasonic pads makes you a good goalie (yes I am), but, It's just an anecdotal trend I've seen.
A common comment I see about guys going from a softer pad (eflex/velocity) and going into a Vapour pad is how stiff it is. The lack of torsional flex throughout the majority of a pad, even for Bauer's soft line, always throws people off the first time they try it out. It can be a make or break feeling for some guys.