Ya sure there are only 4 countries in the EU that allow a contest cause of laws and thats exact the 4 SE got studios in...
It's a lot easier to do things in countries where you have a physical presence. Why should this be a suprise to you?

btw why are other companies allowed to make contests with almost the same price (ticket to tokio for a music show) in every single country of the EU.
Good for those other companies. I can't imagine the work their legal teams had to go through to make it possible. Not to mention the costs- the laws of many countries makes it disporportionately more expensive to offer a prize there than another country. This is a common issue even here in north america, where contests often apply to everywhere except Quebec, Canada because of that provinces' laws relating to the protection of the French language there that substantially increase the cost of doing business there. It might cost SE 2-3 times as much to offer the prize in your country compared to another.

I 100% guarantee you, the offering of a physical prize with real monetary value is the complete, entire reason why this contest is limited to certain areas, and is completely the reason why most contests we get are for ingame item rewards in general.