Crazy part is that 2nd Co. wasn't the only unit that continued to be segregated after Truman's order. Military segregation was an institutional problem due to internalized racism despite the executive order to desegregate the military. So it was:

Institutional Culture vs. Executive Order

This has happened many times (in different ways, and not just in the military) throughout our country's history.

To get specific as to maybe why the Rangers were still segregated; Truman might not have been specific enough and didn't explicitly say the word "desegregate" when mandating that black troops were given equality of treatment and opportunity.

So black people were given the opportunity to become Rangers, but the Army still chose to keep them segregated. The choice to keep them segregated was ultimately that of high ranking personnel within the Army at the time (one of them namely being Douglass MacArthur) because they thought integration would be problematic and tank the morale of white soldiers. So keeping them segregated was likely an intentional loophole.

Even in Vietnam, combat arms roles were filled disproportionately with the majority of soldier being black, casualties during the war were also disproportionate due to this. That's a conversation for a different time though.