Basically because they're not building for muscle car guys. 6 speeds usually have two overdrive gears instead of one like a 5-sp. If the extra overdrive gear is just there to lessen the drop in rpm between 4th gear(1:1 ratio) and overdrive, I don't see the point. I could see this irritating some yuppie driving a lexus but it wouldn't matter to me.
You don't see the advantage of a 6 spd?,.....Dodge Viper for example, 6 spd double OD Tremec T56, with a 4:10 rear gear allows you awesome street performance and 190 MPH top end speeds;...that's the advantage of 6 spds....but you Mopar guys are just warming up to 5 spds,..finally 
,....another 10 years you'll see the benefits of 6 gears 
