With older players getting bored or busier, they quit.
After spending months of building accounts and getting set to rim, many could not be bothered rebuilding and quit.
With shrinking then disappearing grey cities, farming becomes harder also making moving out of friendly oceans harder when combined with a 48hr CS travel restrictions (entire enemy oceans are surrounded and conquered leaving players "land locked") unless they found a city, which takes around a month -back to the first point.
Players reach high city levels and it takes longer to acquire new city slots (although this can be somewhat overcome with festivals).
More experienced players turn to using gold/premium, others can't afford/lack payment means, and quit because they can't compete.
There are probably a few more reasons why Alpha is dying apart from players leaving.
Large alliances and pacts with more than a dozen neighbouring alliances also limits the number of fights in this war game.
Alpha needs a shake up anyway, stuff happens but it isn't that interesting