The compound ncl3 is nitrogen trichloride, but alcl3 is simply aluminum chloride. why

The answer to this question would be:
NCl3 is a molecular compound (two or more nonmetals), and therefore in its name prefixes indicate the number of each type of atom. so NCl3 is nitrogen trichloride.
The compound AlCl3 is an ionic compound (metal and nonmetal), and therefore does not require prefixes. so AlCl3 is aluminum chloride.

Both of nitrogen and chlorine is nonmetal, but aluminum is metal. Metal with nonmetal will make an ionic compound that doesn't need prefixes.