I'm not wearing one because I don't have to, and I don't want to - Hope that is a plain enough reason.
When I am a customer in any shop or indoor area I wear a mask (and have since the start.
(a few more reasons/"excuses")
My customer demographic is skewed to over 65, a lot of whom are hard of hearing or deaf. Me not wearing a mask helps.
I am in a customer facing role in a job where people are often upset or confused and need help sorting things out. Being able to show emotion by not wearing a mask helps.
I am mostly behind a perspex screen.
I maintain social distancing at all times... for my safety. If you wear a mask it protects me if you choose to ignore it and step into my "zone".
I am sure there are more reasons, but the "why aren't YOU wearing a mask" customers tend to sound a bit like petulant toddlers, a bit like the "I'm not going in shops if I have to wear a mask" customers.
That is fine - I am sure some places will be losing custom over it for a little while - but out here in the real world, most of our customers comply with the law, most of them just get on with it, no drama, no fuss, and we muddle along as best we can.