Buying planes will mean purchasing spare parts and servicing in future. Croatia is not the only NATO country that uses Russian/Soviet planes. Slovakia, Poland, Bulgaria (maybe others?) have Russian/Soviet made planes. Slovakia and Bulgaria were servicing their planes in Russia. Recently, Bulgaria decided to service their MIG-29 in Poland.
In choosing planes it comes to know-how and price. Former eastern block countries are familiar with MIG planes. Russian planes are not the greatest but they are cheaper than American counterparts. Much cheaper. Smaller countries cannot afford American planes and services. Also, USA don’t sell their planes to any country including NATO members. Definitely not their newest planes.