So, exactly like most raid-tier intervals, in the history of the game? Again, why is it surprised? ICC was 12 months, Siege was 14 months, Hellfire Citadel was around the same...
Waiting 6 months between raid tiers is perfectly fine, and to be expected. Even if there are a few examples where it was shorter. Rare occasions though.
Sorry that your guild didn't stick around, but that's very typical for first raid tiers on each new expansion, since the 'new smell' fades and people just move on to other adventures for a while. Again, normal, and common.
(also, friendly reminder that an expansion lasts 24 months, frequently with 4 raid tiers, so, 6 months (average) each. If you 'demand'\ask for smaller gaps at the start, that means you will haver -LONGER gaps- at the end, and the reason why we got so many 12-14 months raid tiers as final tiers of an expansion. so yea, let's try and avoid more of those).
reference ->
https://i.imgur.com/hhNXuU9.png (missing most of BFA, similar to Legion though)