Easy, George was attainted and therefore his son lost his inheritance rights.
John Rous, the medieval historian who died in 1492, claimed that after Richard's son and wife had died a year apart, he'd named Edward Warwick as his heir with Edward's cousin John de la Pole, 1st Earl of Lincoln named as 2nd in line. There has been no proof found that it actually occurred though. Edward was created the 17th Earl of Warwick in 1478 after the attainder, and execution of his father. Richard, Duke of Gloucester had no problem declaring the sons of Edward IV as illegitimate, and usurping the crown for himself. If he had really wanted to see his nephew Edward on the throne instead of himself, I have no doubt he would have been able to get it accomplished.