I don't know specifically in Romania's case, but getting random German noble families to marry into your family and rule your country was all the rage back in the day. The famous example is Catherine the Great becoming Empress of Russia even though she was born in Germany (though to be fair, she russified quite quickly). Most similar to your question about Romania would be the newly independent Greek state that rebelled against the Ottomans in the 1830's and got Otto, a Bavarian prince with almost no connection to the country as their king. In that case it was due to outside power brokerage by the Great Powers of the day, and can only assume it was similar in Romania's case, due to similar circumstances regarding their becoming independent of the Ottomans.