I don't specifically know the answer, but I believe it is due to the fact that a scion will yield at most one grafted plant. While the same scion may have 6-10 buds, and therefore 6-10 potential grafted plants. Even if you have an additional 20% failure rate, bud grafting would produce many more grafted plants.

Arktrees

In many cases, the percentage of successful takes is higher with budding than with many other types of grafting. Budding can require less time than other techniques (and is actually easier than many grafting techniques). A wider variety of branch sizes can be used, and, as Arktrees said, there is more graftable material available for budding. Bud grafts are less susceptible to mechanical damage.

With your typical fruit tree grafting a scion of dormant wood is used and grafted onto a plant has recently broken dormancy. With budding a shield with a single bud is cut from actively growing material and leaves removed. It is then grafted to an actively growing plant. Budding can be a nice 2nd chance if you didn't have a take when bench grafting.

In general, the timing for bud grafting is more critical. T-budding, the most common technique, requires that the bark "slip"where the bud is to be inserted. Determining when this will happen requires a certain amount of experience as it can vary considerably from species to species abd year to year.

I'll bet the timing issue may have something to do with some people considering budding more challenging. Of course other grafting techniques also have timing issues, maybe just not as critical.

All, thanks for the info!