Both franchises were created by the same person. Takashi Nishiyama. King of fighters happens to be the spiritual successor of the original Street Fighter game. And both franchises were successful in their own merit. The difference is KoF had a story line and had more emphasis on timing in terms gameplay while Street Fighter had more emphasis on combos. Saying one is a fake of the other is nonsense. Its like saying Quake is a fake Doom.
I must say, 3D versions are worst of KOF. Arcade pixel versions from 94-2004 are always all time favorite. Besides XII & XIII art style looks good. The 15 looks bullshit. Please learn from Guilty Gears Strive.