It’s because it gives guilds the power to control the pipeline of skilled labor and exclude new entrants to limit supply. Guilds are generally run by their members, and the existing members are much more likely to be white and native born than potential new entrants. Their management structure also makes it more likely that prejudices will be acted upon.

When public unions became a thing in the 1960s and 1970s, they systematically excluded Black people, for example.

A federal government can dictate what it thinks morality should be, but in practice cannot enforce morality in a free country.

Even in the US South, the vast majority of millennials and gen Z are not racist. They aren't in political power yet, but in the next 20 years or so, they will be and the zeitgeist of the area will finish its shift (even the people currently in power are abolitionists compared to the previous generations). Travel the world and you'll see that (perhaps outside some European countries) the US of today is just about the least racist country in existence.

Guilds/Unions formed today won't have the same issues as ones from 50-60 years ago because the general view of people today isn't what it was back then.

Guilds and unions are not the same thing.

No matter what the "zeitgeist" is 20 years from now, guilds will still have the effect of artificially limiting the supply of labor to the fields they control, which has been a disaster for fields ranging from medicine to cosmetics.

Craft unions, like in construction, are more like guilds than the industrial unions. You must qualify in one way or another to be a union member before you can get hired by a union company.

But in an industrial union, as an example get hired by an automaker with a UAW contract (and not in a so called right to work state), you are a UAW member.

I think you mean the very early years 1870's and 80' unfortunately the AFL did discriminate against Black and Female Workers.