Because Dr. Huey has always been the rootstock of choice in California and the grafted roses available in most states probably come from growing fields in California.
For a long time, Fortuniana was treated as a rootstock exclusively for Florida roses. J&P even had a division producing its roses on Fortuniana rootstock in Florida. It was called MerryGro Farms. They had a huge sale when they were going out of business about 5 years ago, and it was the first time many of us ever grew roses on Fortuniana rootstock. Some of the biggest roses in my garden today came from that sale. People in the northern states didn't do so well, however. The roses failed miserably in the colder zones.
