If this is the 5 fractions without hormone therapy (rolled out after the successful PACE-B trial), it's limited to T2 G7 (and possibly only 3+4, not 4+3), so they may feel they have to make sure you're still within that category.
PACE-C trial was dealing with higher risk cancer, and adds in hormone therapy, but I haven't seen that treatment rolled out yet.
Standard external beam (20 fractions with hormone therapy) would be applicable for a larger range of diagnoses.