Everything has its price!
I remember the grand kerfuffle when I wanted a cigar lighter socket in a Twotter. Not to smoke cigars but just to run my noise-cancelling headset, one of the early ones that didn't have a battery pack. There I was thinking to just find a source of juice and tap into it like some bush man. Wrong! You needed technical approval and blah-blah-blah... (Meanwhile, for all you Cessna drivers out there that company got a new anti-smoking chief exec who did away with same on his products, obviously not thinking of other things you could power off a cigar lighter!)
Here you already have the bog-standard PT6 starter-generator approved and in place. If you wanted to go for more amperes I guess that would mean finding a new, more powerful starter-generator and getting that certified. Then you would have a problem with spares, some aircraft with the old kit and one or two with the new kit and how do you get your overhauled unit when it's a one-off and you are out back of nowhere and, "Oh, look, they sent us an old model starter-generator instead of the new model," or vice-versa...
I already saw a Chief Pilot staring a hole in the mahogany bar counter after some bright spark had sent us a PT6-28 instead of a -27 because it's thermally adequate, not thinking about that funny gizmo on the nose case that only the Twotter has. (Of course the old engine had been flown to time-ex on Friday afternoon for a quick engine change on the weekend to be back in service on Monday.) I slithered up next to this unfortunate to ask, "So, Pat... Heard any good PT6 jokes lately?" when he did rather exhibit signs of a serious sense of humour failure there, yes.
I think much of the appeal of the Twotter is in its rugged simplicity (who would buy it for its looks, a blind man?) and any upgrades have to detract from that.