The Beach Club and the two DVC resorts (Saratoga Springs and OKW) don't have signature restaurants (BC has Yachtsman's Steakhouse and Flying Fish close by though). As for the Polynesian, the atmosphere there is more casual than the other monorail resorts, and there are four signature restaurants (if you count CRT) and Victoria and Albert's within a short monorail ride, so I can't think that Polynesian might particularly have need of a signature restaurant. And it's true, they do have a dinner show. Not sure Polynesian has the space for one - it was built before Disney started putting signature restaurants in anywhere. California Grill didn't start out as a signature restaurant, it used to be something like a dinner show or dinner theater. I forget what it used to be called, Top of the World or something.
AKL and AKV are not close to anything else and WL isn't on the monorail, so it might have been considered that they needed to have their own.