In response to Lushus and Poch...
1) There is a major difference between AWA and NWA's cases.
When the AWA folded, their championship was already vacant thanks to Mr. Zbysko leaving for WCW. The championship remained stagnate until AWA's re-opening in 1996 where Johnnie Stewart won the title. That is 6 years of inactivity. Mr. Stewart was champion until '99 when he lost to King Kong Bundy and it is not known if he was entirely active, as his OWW profile is quite thin.
When the NWA lost its champion in Ric Flair when WCW left it attempted (see: Rise and Fall of ECW) crown a champion less than a year later. Then, Chris Candido won the championship in another tournament. Henceforth, the NWA World Heavyweight Championship has been consistently to this day.
2) I guess in AWA and NWA/TNA's cases it's their credentials. AWA was resurrected by Dale Gagne, son of Verne Gagne. TNA was originally presented as NWA-TNA, plus, TNA has a contract until 2004.