I wish I knew, I miss the rotary valve. I think part of the problem might be complexity, because I guess of things aren't all lined up exactly right you can start to get some problems. Then there's the bulky dimensions as you said, a rotary-valve engine definitely looks "weird" among a field of piston-port and reed-valve engines, and we all know how much appearance determines peoples' preferences for bikes (just read any thread about a 2009 Maico). But ultimately, I don't know why they died out entirely, I would imagine the rotary would give you great flexibility in tuning to help mold the power curve however you want it. Cam-Am chose for a balls-out explosion of insanity, but on the other hand my experience with rotary-valve Kawasakis has shown a bounty of power just about everywhere. In a straight line, my dad's 250 green-streak is just barely barely slower than a 1980 Husky 250, a bike that is 10 years newer and insanely light-weight by comparison (the green streak feels like a Harley when you're loading it in the truck!)