Very likely the road is closed because it has several feet of snow on it. We passed through Yoho in April and found a tremendous amount of snow along the highway.
We went to the falls in October. No snow but still ice from winter in places.
I drove our 20 foot motorhome up to the trail. Coming down I couldn't make the turn on the hairpin curve. There is a nice turnaround right there for vehicles our size so I stopped near the edge of the cliff, flipped the gear shift to back up . . . And we started forward! It was in low gear for the descent and flipping the handle upward as usual went to drive, not reverse. I'm still alive so you know I stopped in time and made darn sure I was in reverse before trying again.