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Possibly the coil output .
With new plugs, the profile of the center electrode is sharp and square, which allows a quicker (Lower.) voltage/amp discharge across the gap.
As gas errosion rounds off the electrode profile, voltage requirements increase and a weak coil will need more time to build up a charge to spark the plug....which sometimes will be a non spark event...missfire...or even the turbulence in the chamber blowing out the weaker spark.
Another reason why multi-ground electrode plugs have come into use on late model engines so manufacturers can prove/provide no tune up during the DOT mandated emmision complience distance.
You'll find loads of interesting reading if you Google 'spark plug'.
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