The best analogy I can think of is that, in literary terms, movies are short stories, while prestige series are novels.
The traditional movie is a one-off; it tells a story, briefly. It can ba amazing. It can be thought-provoking. It can evoke a world beyond the characters. But then it is done.
The series, on the other hand, luxuriates in its length. The characters become more complex, and the plot, instead of being about a single "event" becomes more of story, unfolding over time.
(I guess this makes the miniseries the novella, but that might stretch the analogy too far!)
I think that the big difference is that, for the most part, until the Sopranos era that we have entered, TV series usually did not attempt to tell a single unified story. They did not aspire to greatness, so much as to comfort.
As a collateral issue, I often run into the same problem when picking something to watch on streaming- for whatever reason, choosing a prestige series that is "one hour" episodes is fine, even knowing that I will binge two or three of them, while choosing a two-hour movie feels like too much of a commitment.