Also keep in mind May's original cameo in 2008 was only 2 years after AG ended in 2006, so it was fairly recently and her VA could still play the character.
Also besides Misty/Brock, most other characters are allowed to be forgotten after they leave the show. Characters in the middle seasons like May, Dawn, etc. will never be given the same treatment as the original cast of Misty/Brock. Don't expect to see Dawn return when DP remakes happen either