I believe it's partially due to because when an alliance starts to dominate a score range. People get upset with constant beiging , and the sitting which lasts up to 5 days if you come with 700 odd planes, and then get smashed instantly and are removed from playing for 5 days, rebuild and do the same thing again for multiple weeks I don't think many people would be every intered in playing long definitely with such stagnating growth which comes in war.
And as said, a lot of people consider this more of a passive game rather than active, and the war requires activity rather than passivity.