I'm going out on a limb now: I dind't think the caps are not really necessary at all during the week. During that time, both LW arenas combined are often at comparable numbers to a single arena on weekends. And if 300 players in one arena is perfectly acceptable and not "overcrowding" at all on a Saturday, I wonder why it should be detrimental to the game on a Wednesday?
Right now it's 1 am local time. Caps kicked in 6 hours ago, and right now the numbers are 184/100 and 104/150. For more than six hours the total numbers are not even remotely approaching "unhealthy" or crowded levels for a large map.
For a US primetime player it may not make that much difference if there are 250 instead of 450 players in the arena. Having to face an arena with less than 100 players instead of 250 is quite a different gameplay experience.
Last tour, I went out and noted the arena numbers for a whole week. This is one example:

(on that day, I didn't record another data point between 2 and 4 am, the actual total peek must have been around 350 players during that time)
Titanic Tuesday is basically the only day of the week were I get to fully enjoy the game on a same level as US prime time, and how it was when - and why! - I joined AH 5 years ago. The only alternative is to stay up until ~4am. And not, because "we" euro prime players are not enough to fill an arena.