Warum haben es Autofahrer aus Russland bei der Einreise nach Serbien einfacher als EU-Bürger?

Both EU citizens (Swiss, Norweger, Bosnier) of course and Russians can enter Serbia without visas. There’s no difference.

But Russians can only enter defacto by plane because by car it would be very cumbersome.

For this, it is necessary to cross EU states. A number of EU countries such as Poland, Lithuania and Latvia have tightened the borders, and the Czech Republic is threatening to denounce cars with Russian marks.

From Sotchi or Sevastopol by ferry to Istanbul, then with another ferry from Istanbul to Constanța across Romania could work but would be an absolute tortur.