How and why has suffrage been expanded in the u. s?

First the laws were changed to let black Americans vote.

Then later the laws were changed to let women vote.

Later the laws were changed to get rid of poll taxes (discriminated against the poor) and literacy tests (discriminated against the poor and blacks).

Most recently the voting age for federal elections was standardized at 18, so in some places that may have given younger people the chance to vote.