The main reason is: Numbers breed numbers. Few people on login screen means even less people bother to join that arena.
But there's is more to it. Several factors contributed to the decline particularly of the EW, (I can't say much about MW):
When the initial craze was over, the majority of players migrated back to LW. Partly because of less limited plane and GV choices (only 1 tak in EW for example), which was even more important as all gameplay settings were identically with LW. So you had field ack, CV & strat puffy ack at the same fierce levels as neccessary in LW, but with much slower, more fragile planes that carried a lot less guns & bombs).
Then the EW was (and still is) the Hurri 2C arena, which quickly dominated the arena in a way that would quickly had put a perk tag on it in LW, but in EW it didn't even go to ENY 5.
Which meant a horde (country drastically outnumbering it's opposition) could swarm you with Hurri 2C, a gross imbalance that never was corrected.
And that imbalance was getting even bigger as player numbers further declined. Some squads did start to make EW their home, but their numbers became a problem. You would usually see country numbers like 20-2-0, as they refused to split up ("Is it our fault the Knits don't come here to defend?") - Which made this arena even more unattractive to anyone not only interested in unopposed base captures). When logging in, you only had the choice to go with the horde against undefended bases, or fight against 10+ Hurri 2C's all on your own.
