Why doesn't the regeneration power make his face like before?
Because his regenerates to heal, not to recreate. He was born deformed.
That's how he looked before he presumably died for the first time:
Plus, his healing and other powers become more powerful after Friday the 13th Part VI: Jason Lives, where Tommy Jarvis's action accidentally revived him and, ...
According to the Friday the 13th Wiki, only Jason X is out of order.
They list the years that each film took place:
Friday the 13th - 1979
Friday the 13th Part 2 - 1984
Friday the 13th Part III - 1984 (A direct continuation of part 2)
Friday the 13th: The Final Chapter - 1984 (A direct continuation of part III)
Friday the 13th: A New Beginning - 1989