stpe side bed question

[Rod Run]

I have a '71 step side but it has a '73-up bed on it.

I know that the tailgate is interchangeable between the two different styles, but does anyone know why makes the bed itself different?

Why you can't just bolt the earlier fenders onto the later bed? If I could do that, things would be so much easier

Thanks in advance, for any help or answers that might clear this up