It is because in those days when you became NWA world champion you had to pay a $25,000 retainer so that the belt became your responsibility. This was basically to make sure you looked after it.
WCW took over NWA and with it had Ric Flair and the belt, of course Ric stayed as the world champ. WCW was ran by people who had to this point no involvement in the industry, and didn't know about many of the traditions there was such as the $25,000 retainer for the belt.
Ric Flair didn't get along with the WCW management especially the booker Jim Herd and was eventually fired and asked to hand over the belt. Ric said the belt was his untill they gave him back his $25,000. They obviously refused to pay it as this money had been originally paid to the NWA and not them.
So Ric kept the title and took it with him to WWF.
I'm not sure if he ever got his money back but he obviously gave the title back in the end.