Haven't been on in ages, read one article, want to contribute this:
The point of league challenges is to promote bottom-up growth. TPCi wants you not to complain about lack of access, but to instead apply to become a TO and host things in your area.
The reason not all leagues can host LC's is because the league needs an active TO with a good track record of hosting events and reporting results in a timely fashion. That's the reason some leagues don't have LC's and some do.