My guess is Leves where seen as the perfect casual group event.

You can solo them if you have less then an hour.
You can use the party search and instantly join and warp to a group anywhere in the game and team up on them to get more sp.
IT is the best SP in the game so the hardcore players will join up with the casuals.
With thousands of people doing them a day and there being 10-20 partys seeking at all times, you can log in instantly party up and play.

So it is endless partys which is easy to join and can be 30mins long or 4hrs long depending on how long you want to play... really the perfect casual party setup.

Except no one uses the party feature which killed half that idea. And the population being about 10% what they expected also dealing a blow the system just does not work as intended.

This above would be my guess why the game is focused around it. Tanaka wanted the game to be party based (which every other mainstream mmo is not) and casual at the same time.