WHY IS ZONE EFFECTIVE?
1. Any time a defense can force its opponent to run its zone offense; they have achieved a little win.
2. Offenses should dodge against zones, but offensive coaches continue to think a lot of passing is the way to beat it, so they run scripted plays. Really, zones are set up to be in position for quick passes.
3. It gives the chance to play a more help-oriented, packed-in defense rather than find themselves on an island.