I think know-how is major factor in choosing planes. Former eastern block countries had many MIG planes. I am not sure if these countries had SU planes. If they had them then not in large numbers. MIG planes lost competition to SU planes. There have not been major development and releases of new MIGs in the last 25 years. Yet, Croatia chose old MIGs.
Also USA won’t sell war planes and other military equipment to many countries including NATO countries. Instead, Americans will offer American military bases, so that these countries are completely reliant on USA.
Russia is attempting similar tactics against Belarus wanting to build air-base in Belarus. Strutegitcally, Russian air-base is not needed in Belarus. Russia already has bases in western and Russia and Kaliningrad. The president of Belarus stated “we don’t Russian air-base. What we need is weapons to train our own personnel”.