What I think:
1) Sedan market is bigger than Coupe/Cabrio market
2) M5 is the best sedan on the market so it gets a disproportionate share of the high end sedans
3) There are many other competitors in the coupe/cabrio space, some of which are quite credible. I wouldn't say the M6 is the best coupe/cabrio - Maybe cars from Porsche, Lamborghini or Ferrari can claim that. But regardless of your budget, you can't beat an M5. So M6 doesn't get a disproprtionate share, and is has fiercer competition in a smaller market.
2014 Porsche 911 Turbo S
2015 Mercedes-Benz C300 Sport 4MATIC