I used Les peuples, but it was replaced with les gens. why is this?

"Les peuples" would be "the peoples". That would be referring to populations:" les peuples du monde".

If you're talking about a group of people whose individual members are identifiable, "les gens" or "les personnes" is more correct.

I think it is because 'les peuples' signifies 'the people' as a collective concept e.g 'the people revolted.' Whereas 'les gens' signifies 'people' as a collective group of individuals that act independently e.g 'some people like this'.