There could be many reasons why BMW is increasing their market share while others are losing. However I believe the same is true of Ducati, Triumph and Victory as well.
I think that rather than it being a "rich man's market" it has more to do with the perception of value. It may be that when an economy tanks that the people who do have money enough to buy an expensive non-essential like a motorcycle want something that they perceive as holding its value and of high quality.
Perception is a funny thing, it may or may not hold up in the harsh light of reality.
It may somewhat explain why this occurs though and why companies like Suzuki have to heavily discount product to move it. Or, Kawasaki for that matter, I bought my Concours for $3k off list as it was a 2012 and the '13s are right behind. Its a very high quality bike but our local bmw dealer continues to sell out their stock, almost always at msrp.
'14 HD FLHR
'01 HD FXD
'09 HD XR1200
'79 BMW R100