Rockstar probably feels that the 360 and PS3 versions are now crude first-efforts and don't want to release GTA V close to that state again. It's a branding issue, it would hurt the prestige of the Grand Theft Auto brand to receive a down-port on an ancient mobile chipset. GTA V has been remastered for more powerful hardware, and Rockstar thinks those versions are representative of what GTA V is now.