Why are aircraft not equipped with nose cargo doors when being converted from passenger to cargo variant?
The simple answer is there isn't the space. The problem with nose cargo doors is the cockpit. The cargo door must be high enough on the aircraft to line up with the main cargo hold to be useful. On larger aircraft designed with this in mind from the start, (747, C-5, An-124, An-225) the cockpit is placed in a location above the main deck so that the cockpit does not obstruct the path from the main cargo hold to the nose door.
This is specifically why the 747 was built as a two deck aircraft with the hump. They wanted to be able to fit a nose door, so they needed to put the cockpit on a raised deck, which left them with the hump deck for aerodynamic reasons.
However, on current generation narrow bodies, they simply aren't tall enough to do this. They are single deck aircraft, with the cockpit located directly in front of the main payload deck.