I highly recommend a NYT article on the subject by Debby Applegate, who won the Pulitzer Prize for her biography of Henry Ward Beecher. (If you read the whole article, which you should, let me note again that, when Stanton referred to the “lower orders” of men, she was talking about the poorest and least educated. Socioeconomic class became entangled with racial and ethnic bigotry.)
... there was no intrinsic reason both blacks and women couldn’t attain the rights of citizenship or suffrage at the same time.
Of course, it wasn’t black men or white women who decided that there wasn’t room for them both to enter. After all, neither group could vote in the ratification process. Stanton and Douglass may have had a lot to say to each other and the press, but neither of them had any say in the wording of the amendment.
Instead, this was decided by a coalition of Republican politicians in Washington who supported black suffrage — and thus the creation of a sure new population of black Republican voters — as a way to shore up their precarious majority in Congress. (There were nobler motives as well, but the timing of the amendment was all politics.)
The exclusion of women was also a partisan decision, since enfranchising white women would run the risk of creating as many new Democratic voters as Republicans. The Republicans’ public line, however, was that the amendment would have no chance of ratification if it were so bold as to offer universal suffrage.
… The 15th Amendment marked the end of the public’s commitment to major social change. Within the decade, the Republican Party had shed its progressive activism to become the party of big business and laissez-faire policy.
…. if history offers a lesson here, it is not that Americans cannot handle too much change at one time or that we must inch our way, one by one, through the door of equality. Rather, it is that opportunities for genuine change are rare and when they occur we must kick the door off the hinges while we can. It is much harder to pry open the public mind once it has shut itself up again.