Anime Viking writes:
Progression of the plot is clearly of low importance for Sword Art Online as the show has become increasingly stagnated within recent episodes.
Another plausible explanation is that the show is just finding it harder to hide how incredibly shallow its plot is. At least with SAO, the hundred bosses on each level create the illusion of a longer story.
ALO, on the other hand, can’t hide the fact that a simple rescue mission just isn’t enough material to fill an entire season.