Why is Montana taking so long?

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.