Many reasons. Can you be more specific. What exactly is happening? It could be a bad starter. It could be something wrong with your fuel injectors. It could be an issue with the spark plugs / distributor. It could be that the switch that is activated when the car is put into park or neutral is broken if it is an automatic. Or if it was a manual it could be that the switch that the clutch activates when pushed in is broken. The list goes on. You need to be more specific.
The 2001 Nissan Xterra has 16 valves.
The 2001 Nissan Xterra is a gas-powered vehicle.
The 2001 Nissan Xterra has an inline 4 engine.
The 2001 Nissan Xterra has a 5-speed manual.
The height of the 2001 Nissan Xterra is 6 ft. 2 in. (74 in.).
The 2001 Nissan Xterra has double overhead cam (DOHC).
The 2001 Nissan Xterra runs on regular unleaded.
The 2001 Nissan Xterra is 14 ft. 10 in. (178 in.) long.
The 2001 Nissan Xterra has 37.8 in. of rear head room.
The 2001 Nissan Xterra has 38.6 in. of front head room.
The 2001 Nissan Xterra has 32.8 in. of rear leg room.
The 2001 Nissan Xterra has 54.4 in. of rear shoulder room.