I can only speculate. I think that metal sells well because Croat metalheads like merchandise while average Croat the music enjoyer doesn't care for it.
Music piracy for personal enjoyment is basically legal in Croatia. If you're cheap, or on a tight budget, which covers majority of Croats, if you don't have specific passion or allegiance to the author, there's little reason to go out and buy anything.
So - Maiden sells well, but most other music undersells heavily. And we're talking about a tiny market where promotion and exposure counts as gold.