USA Today give some detail of the tortuously long count in Montana. The reason for the long count is explained by the local election law as described Associated Press:
A candidate in Montana can request a recount at his own expense if the margin is within half of a percent, which would be a margin of roughly 2,000 votes in the Montana U.S. Senate race. If the margin is less than one-quarter of a percent, the state and counties pick up the tab.
With one precinct still out, the margin is about 1,700 votes.
Apparently provisional ballots might also cause a delay to the declaration. If these outweigh the margin between the two leading candidates, there might be further delay.