In general the overall playbase quality has dropped drastically. A wave of older players have quit because of the gear system, as well as the attitude of players nowadays.
Why don't people interrupt? because it 'costs them dps'. They have to go out of their rotation to use an ability that does no dps. This is assuming they have it ready on their actionbar. It's the same reason why the mob youre tanking always dies last. the DPS unload on the mob with the most hp, because they want to top meters.