Why were so many arcade to c64 ports different between America and Europe? Why taking the time to develop two completely diffefent versions of the same game?
Different television/electrical standards. America was 60Hz/NTSC, Europe was 50Hz/PAL. Games written in Europe on the PAL standard were usually incompatible with NTSC machines, and vice versa. Especially so if the game used advanced programming tricks that relied on exact timing.
Usually the PAL version of a given game is better, the C64 scene was just more developed in Europe. Excellent example being OP pic, Bionic Commando.