my question is simple why Nitrocellulose does not explode since it contains Nitrogen and oxygen in the structure of the cotton why it does burn ?
It does detonate in certain scenarios.
Detonation is a case of rapid decomposition where reaction front travels at supersonic speed. Obviously, for such decomposition to occur, the compounds needs a way to decompose with big enough release of energy. Also, in this case decomposition front travels not by means of heat transfer, but by means of pressure wave. This means, that the detonating solid must be, well, solid and not fluffy. Another requirements is generation of the initial pressure wave. Some compounds can create it on their own, some require a primer and some require a pretty damn big primer.