Rugby League and gridiron have a shared history both are the next step up from union. But evolved in isolation. It is most likely League copied early forms of gridiron. The 6 tackle /play the ball rule probably came from rugby league people visiting north america. Rugby League is well received in the america's by gridiron communities who see it as amateur form of gridiron. But it does serve as a warning for us with superleague/nrl as we upsize and concentrate on physicality and de-skill our players.
Gridiron had limited the number of tackles / play a long time before league did.