Maps selects this route because it is shorter. It may also have the estimated speed off, and also calculate it as faster.
On GM Streetview I see a warning sign (not especially big, but easily read on SV).
WARNING
TRUCKS, MOTORHOMES,
BUSES AND TRAILERS
PROHIBITED
ON OLD PRIEST GRADE
USE HIGHWAY 120
NO (4 slash circles).
Before the turn off there's a large green direction sign, 120 to the left, Old Priest Grade to the right. And a 7500lb weight limit. The main highway striping goes left.
Chinese Camp to Yosemite Lakes is 32.4m, 46 min via Old Priest. Dragging the route to 120 changes it to 35.1, 52 min.
So new grade is on the other side of the canyon, following contours more, longer and thus climbing at a gentler rate. From a sampling of Streetview images on Old Priest I can see the New across the valley.
And from the of New Grade looking toward the upper junction:
https://goo.gl/maps/n6ZsuPCw8z32Another hint as to the grade, even on New is a sign "Turn off air conditioning next 5 miles". New has some curves marked 20 mph.
