I think more of it is simplicity than anything. Can you imagine a roadie trying to get you dialed back in on a Mark IV? Imagine this... It is dark, you need a quick adjustment, ham-fisted Larry your roadie goes to give you a little volume change so you can hear yourself better. WHAM!!! You kill the sound guy, your bandmates, the first five rows, and yourself, not to mention Larry, with the blast from his suggested little adjustment. He couldn't help it, it was dark and his flashlight batteries were low. You just got done yelling at him the last show because he was on stage too long. So he thought he would quick slip the knob what he thought was enough. Turns out it was too much. Or this one is good too... Having no success kicking people back off the stage from the riot you are ensuing and not wanting to swing your guitar you pick up your Kong Kontroller and knock someone out. You get arrested for assault with a deadly weapon. Then you get sued for every penny and all your property plus 10 million. Or how about this one... Some fool backstage decides he is going to run off with your Roadster because he can't afford one. He has never seen one in person and the temptation is too great. So he swipes it when noone is looking leaving you hanging for the next gig (tomorrow night). This is funny, don't forget this one... After hearing you sound check with your Stiletto, the guys in the other bands beg you to use your amp for their set. After denying them as you should and telling them to play their Marshalls, they all jump you and buy you a one-way ticket to the ER. Your amp gets played anyway and destroyed as they fight over it after the show. You and your amp are never the same. These are some reasons not to gig with these amps. :wink:
All seriousness aside, I think it is because that is what backline companies usually have sitting around. I also think that the Dual and Triple are the most popular and most often sold amps Mesa builds. They certainly have the easiest controls to learn. They have also been around the longest with exception to the Mark IV. Many have yet to completely master these other amps also.