Because people from the US lost money from his activities.

Would you want some say Eastern European crime organization to steal your identity, ruin your credit, empty your banks and make it extremely difficult for you to live while your country can't do anything because they can't arrest the person if they know who it is, or influence their government to stop them?

That is why extradition exists for governments, tons of countries have extradition treaties with tons of other countries.

I had my identity stolen, had to freeze my credit, change my cellphone service, call banks, get a new social security number ect. The person who was from the US then sold my info to someone or some group in Belarus and I had a lot of issues from that as well. It was complete hell for 2 years, constantly being on the phone with US government services, international banks, US banks, US and International cell services.