IDOT not using Marion, IL on either I-57 or I-24 is a bit inconsistent, or possibly even Marion-Carbondale (especially from Mt Vernon coming south)
Pretty much all the other Interstate Junction cities along I-57 (other than South Suburban Chicago @ I-80 and I-294; D1 is a little different on their Control City application anyway) get Secondary/Local Control status, in addition to Cairo
Heck, I-24 West (North) has a signed Control of Interstate 57 in IL (with a slight acknowledgement of the St Louis Control right before termination @ I-57)
Is it because Marion is mostly just north of the interchange, whereas Champaign, Effingham, and Mt Vernon are right at the Interchanges? Marion does have some land area around Lake of Egypt, which is very close to the 57/24 interchange
If Cairo (and East St Louis on 55, 64, and 70) warrant Secondary Control status, so should Marion IMHO