We wish there were more breeds that made good Service Dogs, but other breeds don't have a good track record. Personally, we LOVE each and every AKC dog breed, but...each breed was originally created for a specific purpose and their resulting characteristics don't match the requirements of a Service Dog.
The Service Dogs we choose to train need, for our program, to have a rare combination of: A. Low energy, and B. High intelligence. This combination is not common in dog breeds (smart dogs tend to be high-energy, while low-energy dogs tend to not be very trainable).
In addition to those requirements, Service Dogs also need to have the following three attributes: A. Have low prey drive, B. Be good with all strange dogs, and C. Be good with all strange men.
If all these requirements aren't enough, the breed also needs to be the right size because: A. Small dogs can grow fearful from being stepped on in crowds, and B. Overly-large dogs are difficult to put under tables and out of the way in airplanes and buses.
That's a lot of requirements! For the above reasons, we have found that we usually work with Labradors and Goldens (and mixes of those breeds).