The decision to not make Volume 3 is personal and has everything to do with Uma. Where am I getting this information? From him, based on the known fact that his biggest fear is becoming a hack that 98% of all filmmakers one day find themselves in. If you haven't seen his interview with Charlie Rose, he goes into great detail about Directors he admired and then just falling off the cliff and how he doesn't want to go down that road. This is why I still hold out hope that his vanity will kick in and that he will swallow whatever pride he has about Uma and make the 3rd Installment. If he doesn't make KB3, there will be a huge hole in his filmography and that would be worst than being the hack he fears of becoming one day. I don't think it would sit well with him when film students start posing the "what if" with regards to poor decisions he makes about movies he makes.....and doesn't make. As you said, he is born to make this type of genre film, more specifically....Kill Bill 3.