JohnT and others speak truly. It is largely a historical thing. All the curative radiation modalities didn't take off until early in the 21st century when the curative power of dose escalation, hypofractionation and the technologies to deliver it safely became available.
I want to reiterate what a bogus argument "you can't do surgery after radiation" is. Salvage radiation is entirely possible after radiation if there should be a local
recurrence, but recurrences after radiation are seldom local, because it has already treated the area that would be treated by salvage radiation after failed surgery. Salvage anything
has cumulative side effects. (See:Why salvage radiation is bad.
You should not choose any therapy because of the salvage if it fails because with your Dx, all
therapies are highly likely to be curative. It is then a matter of choosing the one that is most acceptable based on side effects, or other psychological factors.
You have to seek out expert opinions from each radiation specialty: SBRT, HDR brachy monotherapy, and LDR brachy. I think you are wise to give protons a pass unless the money and time is no object to you.