Because it was a song he sang on the Zvezde Granda competition back in the spring of 2007. I guess his fan base was just starting to take off around then and the girls liked the song and the word and adopted it for themselves. The reason why the titre was Russian was because it was an original Russian song called that. But a Serbian singer sang that song in Serbian, but kept the Russian word "Krasiva" for the titre and within the song's text.
The Russian version was sung par Valery Meladze and this is it:
link Then it was sung par Zdravko Colic who is one of Milan's favori singers from the Balkans. Here he sings:
link And then here is Milan's appearance during the Zvezde Granda competition which was in Spring of 2007.
Milan Stankovic
link 
