Short answer is Regulation.
As for the pratical side it is a safety thing. You can been seen and identified at a geat distance wearing Blaze orange. Camo in a nut shell breaks up the solid color / shape to blend in with the back ground thus much hard to see.
Why one wears one over the other when not required? Many reasons - Fashon, not to be seen, to be seen, take your pick.
From a deer stand point, this is how I understand it. Deer only see gray, so blaze orange to them is just like us looking into the woods and seeing they gray of the trees.
I have a camo blaze orange hoodi that I use during gun seasons as a pull over. Plently of orange to be seen but broken up with a black woodys pattern to not look like a big pumpkin in the middle of the woods.
Deer are all about movement. If you remained still enough they will not be botherd if you stood in the woods buck naked. They would take notice but generally go on about there business as long as you are still.
That said I tend to agree they are a lot smarter than advertised. They may only see gray, but they know their "grays". I have watched deer while bow gunting in regular camo and they never even look my way as they walk by. During gun season wearing my camo orange within seconds of coming into view they are looking my way. They know blaze orange is a not a natural gray to them. The trick is to not move so they are not aware "you" are not there