For one thing, it's a Norwegian company, which explains its popularity in Scandinavia, and could mean more focus and expertise on internationalization. Second, some of its features are more suited to Eastern European markets. Computers tend to be older and cheaper, and Opera works well in low-CPU/low-memory environments[1]. Many connections are still on dial-up, and many ISPs charge by the megabyte, and Opera has a number of options for saving bandwidth, like disabling images or using a proxy compression server. These advantages are probably less important nowadays than 5 or 10 years ago, but by now it's an established player, which helps.

[1] On machines with lots of RAM it tends to be a memory hog, but it works perfectly fine in tighter environments. They claim their memory strategy changes depending on local conditions.