Han was already a Captain of a ship and crew, capable of that kind of complex 'Generalling' as demonstrated by supporting Luke's run on the trench.
Luke was a flyboy kid off the farm, albeit a savant.
Edit. And further into the lore, the rebels would have needed people capable of leading other large groups of people to success. So they would tack on promotions or immediate ranks to retain that talent, but at levels they could use and trust.
Promoting a savant to General and giving him whatever groups of troops to achieve goals, ia much riskier than promoting the already somewhat proven Captain.
Luke was lined up for General in the short term anyways, iirc. At least in the EU canon.