Why were African-American soldiers mostly excluded from frontline combat roles?

I know the baseline answer is racism but I'm trying to understand the specific logic behind why they were mostly excluded from frontline combat roles.

My expectation would be that racist attitudes would make them be seen as expendable or disposable as in many other cases, racist attitudes led directly to minorities being seen as disposal or worth less than others, such Tirailleurs Sénégalais by France in WW1, The British Empire with Indian troops and the USSR with ethnic minorities and 'undesireables' in penal battalions.

In those cases, they were often pushed into high-casualty roles precisely because they were seen as disposable.

Contrasting this to African Americans in the US military, they were mostly excluded from frontline combat roles but played a vital role in logistics as seen in The Red Ball Express and as well as limited combat roles like the Tuskegee Airmen and 761st Tank Battalion where they proved capable in combat roles.

It's still discriminatory but even in the combat roles they were allowed to do, they didn't seemed to be used as cannon fodder instead undersupplied but given a fighting case.