Good questions. Seth makes the primary point which is just the sheer geographic size of the US. Secondarily, yes, we do love our playoffs.Forgive my non-US viewings here, but please explain to me (and just because I'm genuinely interested, not a backhanded response): why is it that at each regional level they need geographic regions with a championship game at the end? If you're reducing each level to a smaller area so the travel costs are smaller why not have each "conference" just be an independent league? If it's the need for a play-off series at the end for the excitement, you can have that within the league itself rather than needing to play the champion of another conference. Or is it just about the innate "thing" Americans have for conferences?
It just seems to me that if you're going to subdivide so that at some level, whether it be D2, D3, D4, so that you don't have one single entity organising all of the different leagues in a tier, it would be far easier for each independent league to only organise one "conference" rather than, at every single tier, a league always having four conferences. It makes things simpler, and it avoids risks such as the league constantly feeling the need to rebalance because one of its conferences is too strong, or too weak, or is generally not running like the other ones are - ultimately those differences are going to appear anyway as soon as a tier is not all organised by one body, so why not just let the conferences be independent?
For the record, I can get behind the idea of conferences at the highest levels, where it's needed to reduce the travel costs at least to a degree (although the practice will always be to some extent alien to me, and I'll never be persuaded it's the optimal format for a league). But once you get down to a level where the teams are able to afford to travel to all of the stadia within the league, why have further conferences?
In the system I outlined let's call these leagues MLS/USL/D1/D2/D3 etc.
On the east coast where populations are highly concentrated you could maybe get to a place where distance doesn't matter much by the time you get to D2. But the western regions would need to get to at least D3 and maybe D4 before they were fully done with plane requirements.
The regions within each league level just reduce those requirements and associated costs.