You asked what the reasons could be, I gave my best guess. If you keep up with the lists you know that you get a range of responses from folks looking for this service, some get the open arm treatment, others can't find anyone who will touch the job. I think it is reasonable to imagine that the reasons at one end of the spectrum are not the same at the other end.

You can nitpick the specifics, but I don't think it is probably too far off the mark to say tool wear, liability, competition are likely responses. I agree that the flux comment is a little weak, but without spending too much time drawing pictures I think the conditions you could run into from the average home builder could be different than standard commercial work. I once agreed to turn some guitar forms for a guy. He was given instructions on where to get the ply etc... But he showed up with MDF that was extremely nasty to turn. Of course I ended up doing it anyway. No biggie but it's the reality of leaving the well trodden path. If he comes in with nasty vitrious flux, I wouldn't want it near my stuff.

Liability is for real, and two big names, Ritchie and Nahbs arbitrarily slammed the door on the non-pro this year. If they don't want to touch us why should the local shop be any different.

None of this applies to the OP's post if it is true the shop was happy to comply.