Why would they release in the US when people in the US go for phones on a contract? The contracts out for the Lumia 820 and 920 already make a compelling case for the phones, introducing the 620 would only eat into those sales.
In other markets, less people go on contracts with phones or those contracts are simply unavailable, so they buy their phones outright. With the 620 being about half (or less) the cost of the 820, it makes a good proposition. (about $500 - $600 for an 820 and $700 - $800 for a 920 outside of the US, btw)
It'll probably come to the US once they've sold enough of the 820 and 920.