Hi Rikky,
I was 4+3 possibly T3a and had an operation. The tumour was said to be at the apex which is away from the bladder. Perhaps yours is nearer another organ and RT can get a wider range.
Perhaps if they decided yours is a definite T3a and 4+3, which is a higher risk, then that's a risk too far.
Mine turned out to be 4+4 and T2a after they removed it. I wondered if he'd have operated if he'd known, although it being negative margin as well lowers the risk presumably.
On your profile you said you were quite fit and low BMI so I can't think of another reason except perhaps the surgeon sets limits on what he's willing to do and overall the MDT decided your best option is RT.
As you're well down stream it's probably better not to ponder too much on these matters really. You are where you are and looking forward is enough for anyone to worry about. Although no harm in asking if it's not causing you more worry.
Hope things are well,
Peter