My top 5 reasons are:
1. You don't source your ingredients locally or seasonally, preferring to buy from a multi-national wholesaler who only cares for profit and not the amount of carbon their trucks destroy my planet with. Because you don't buy your ingredients seasonally you customers have the choice of bland monocultured fruit and veg that's got as much nutrition as the nasty plastic packaging it came in.
2. You don't care about animal welfare, preferring to buy battery farmed chickens that can't stand up properly, that suffer horrible diseases and live in virtual darkness. The eggs these poor creatures produce taste crap and they have to be fed on coloured feed in order to produce a yolk that looks yellow. I could also mention all the other animals like veal calves, but I think you get the picture.
3. You don't recycle the waste your restuarant produces, preferring to stick it all in a plastic bag and hope the landfill won't contaminate the water you drink. You could instead compost the fruit and veg waste, recycle the cardboard, glass and PET plastic and return the shipping boxes back to the wholesaler to use again. Bit more work, but...
4. You waste energy. Why aren't you using low energy light bulbs? Why haven't you replaced that fridge with a low energy one? Why haven't you turned the thermostat down 1 degree? Think of the money you'd save - oh, so now you're interested...
5. You are still serving cod and other endangered fish. Your argument is "it's what the customer wants..." So what are you going to do when those species have gone? Over fish another one?
Of course, you could say "So what? I don't what this guy as a customer", but what if I'm not the only one who won't eat at your restaurant, what if there were more, hundreds even, would you care then?
What's it going to take?