Because "No" is the most overused (abused) word in attempted dog training

Fact is, you'll never get far training a dog if you're telling it "no" all the time.
We
people find it easier to remember that what we actually need to be telling our dogs is what we
want them to do, not what we don't want, when we use a marker other than "no". The dog isn't going to care - they don't know what "no" means anyway. But it's very hard for a
person to remember that "no" is nothing more than a marker, after which the correct behaviour must be demonstrated and rewarded.
And just to add to all that - people go right ahead and use "no" when they actually mean "leave it". AND when they mean "get off". AND when they mean "don't bite". AND when they mean "come here". Get the picture? No isn't a command - it's a catch-all for all the things you don't like but aren't giving the dog clear instructions about. LOL - the end result is just a frustrated and confused dog, combined with an enfuriated human who doesn't understand why their dog won't listen to them.
Bottom line is that training is far more effective when you concentrate on what you actually want the dog TO DO. Not what you don't want. So if you want "off", well, tell the dog "off" - not some airy-fairy "no" that it doesn't understand.