I am guessing like you, that there were a combination of weight and clearance issues. If I remember correctly, the one story I heard of a K27 being run through was with a minimum of coal and water to keep weight down.
As for why not upgrade, there WERE a number of bridges, and compared to the rest of the NG circle, not a tremendous amount of traffic, especially generated on line. No mines that I know of. Cattle and sheep at certain times of the year, perhaps some other ag crops at Cedar Creek.
I'm glad it stayed the way it did... if it hadn't, what are the odds we'd have as many C-16s, C-18s, C-19s etc as we do ? no, the C-21s didn't make it, but the rest did.