I completely agree with your remark about manner of release: I think so much of what made TV fandoms so big is not just the sprawl or imperfections, but the (previously) inherently serialized nature of the storytelling. Even in shows that have miniseries stylings but still release in "seasons" (Sherlock is the one that I remember this most clearly in) seem to have similar upwellings of content during those breaks. One sees this across the biggest book fandoms as well -- was not a heyday of the Harry Potter fandom in the enormous gap between OOTP and the last two? Did not Twilight in part thrive because of its nature as a series?

It provides a built-in timeline, gaps between installments that foster speculation and curiosity, and of course, the gap-filler!

I think there may also be something to be said about the choking press of the adaptation/remake -- it's hard to get as excited about something you were excited about when they did it better the first time ten years ago. (Good Omens may be the exception that proves the rule).