Because as I said, I don't get the point of professional-grade signature instruments. If you're a professional, you probably already have multiple professional-grade instruments built to your own specifications, so all these especially Mark's acoustic guitars which cost like a car have absolutely no meaning to me, other than simply pleasing die-hard fans and collectors. I understand that the point is to "play the same guitar the artist plays", but you can't play like the artist anyway, especially MK, so what's the point.
To me, a signature instrument means the artist endorsing a brand, a guitar, preferably an affordable one, for people to learn how to play, not for collectors to fight over it. Our man has stopped signing guitars because people get overly crazy about it, at the same time launching a line of guitars with a price of an apartment. Luckily, a lot of artists get that signature instrument must be affordable, and Joe Bonamassa, Brian May, John Mayer, everybody sells affordable versions of their instruments.
If I want to play like Tommy Emmanuel, the last thing I want to do is to go out and buy his $10,000 signature Maton 808 TE Personal acoustic. No, thanks, a "regular" Gibson will do...