The original trilogy was a huge box-office success that has garnered a cult following, so my guess is that the filmmakers were trying to profit from the original. By changing the sport from karate to kung fu they are also targeting a newer audience (kung fu has very much become a 'glamorized' sport in film today in my opinion) and giving the films a fresher approach. On another note, since the originals are still so well-received and adored, I also believe that the filmmakers would have been less inclined to do an exact remake as that would have been met with a lot of criticism (no one likes it when Hollywood butchers a classic).