At the time, LW was among the only ones that could handle the influx of registrations.
So naturally, it became the default one, as people would want to get on the biggest one, similar to a way the biggest Mastodon instance is very prevalent.
People were also afraid their All feed won’t be as full if they were not on LW.
Nowadays I think the repartition is a bit better, and most of the top communities have at least an equivalent out of LW.