The reason for the decline, in my unsubstantiable opinion, is twofold:
1. The product is poorly marketed and relies entirely too much on word of mouth.
2. The number of multiple teams held by individual coaches has declined, so the replacement rate due to issue #1 is lower than the turnover rate.
A bigger issue looming in the future will be getting coaches to join worlds when there are only, say, 8-10 seasons . Replacing turnover will be brutal then.
My two cents.