http://www.dot.ca.gov/dist6/opsnowflake/guide.pdf
Why are chains not allowed on the Grapevine, as is the case on I-80 over Donner Pass?
Since the snow and icy conditions are often limited to the summit, a location for the removal of chains would have to be made available and this is not viable due to the traffic volumes. This can make driving all the more dangerous when it does snow because drivers who don't travel through the Grapevine regularly would not be carrying chains, not expect snow, nor have much experience driving in it.
Other differences between the two passes are the grades and traffic volumes. The Grapevine is 6% grade for 5 miles, while Donner Pass is 3-6% grade for 30 miles. The graph illustrates the steepness of the Grapevine, over a shorter stretch of roadway as compared to Donner Pass, in addition to greater than double traffic volumes.*