Founder effect. Also, it exists in relatively high frequencies in Ukraine and Belarus (where it also has its peak diversity).
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Watch Series. Rome: Rise and Fall of the Empire
It explains how Germanic people prized most Italian women whom they took as a war bounty.
They did the same to Balkans, took our women, so that males from Balkans had to take their women.
After Dalmatian Illyrian cavalry returned from Persian wars as a part of Roman army, they had no women... and went to hungary to get some from Avars.
Someone will laugh but that's how the world worked.
There was a half Jordanian (maternally Russian) on TA and his Y DNA was I-M223 (I2).
Because they didn't encounter founder effect like dinaric Slavs living outside of feudal system. I am fairly certain migrating Slavs had lower I2 but it expoloded after settlement in dinaric alps and baby boom among free Slavic sheperds.
Slavic I2 is 25% in northern Ukraine (Polesia), most Slavic region on earth.
11 million in Russia and Ukraine.
https://i.imgur.com/3rGwa1s.png