The Bernese mountain dog is a large, friendly, powerful breed. It has been used as a working dog for as long as it has been around. One particular job that the breed has taken on in the past is as a guide dog.

The Bernese as a Guide Dog

When you think of guide dogs you mainly think of Golden Retrievers or German Shepherds. Very rarely do you think of Bernese mountain dogs. However, the breed has been used before as a guide dog and in some cases they can be really good at it. However, mainly they are better suited to other jobs.
The main reason why the Bernese is not better suited to guiding people, is because of its maturity. As large breeds go, the Bernese does take quite a long time to mature. They usually do not calm down until they are around 3-4 years old and until that point they can be extremely boisterous. So this means that it is not always safe to use the breed as a guiding dog.
Generally guide dogs need to have a calm nature. They need to be able to think for the person they are guiding and nothing should distract them. So with an excitable Bernese, it would be difficult to train them not to go up to strangers and generally behave like a puppy. That is not to say that it cannot be done, but generally it is a lot harder to train the Bernese than it is with a Labrador retriever for example. So usually they will not be chosen over other breeds to be trained as guide dogs.
So if you are thinking of training up a Bernese mountain dog to be a guide dog then you need to know that it will be a challenge. It is not something that just anybody could teach their Bernese to do. Even specialised trainers have difficulty training the Bernese to be a guide dog and so for the average owner to do it, it would take a lot of patience and it still may not happen.
Overall the Bernese mountain dog is much better suited as a family pet. Whilst it is a working dog, it does take a lot of specialised training to turn the breed into a guide dog. Only a select few Bernese dogs will ever make it as guide dogs and so that is why they are hardly ever considered for the job. It makes more sense to choose a breed which is better designed for the role of a guide dog, rather than waste time on one which may not make it even after all of the training. However, some select Bernese dogs do have the capability to be guide dogs, they are just better suited for other things.