A couple of them were my omission as the list was much longer until I edited it like Cinderella and Summer Stock. Below is a link to the entire 1950 listing on AFI. Born Yesterday had its general release in 1951, so I guess that's why AFI didn't include that title.Born Yesterday is not on that list and should be. And Summer Stock and Three Little Words deserve just as much a place on the list as Young Man with a Horn. And don't forget about Disney's Cinderella.
http://www.afi.com/members/catalog/SearchResult.aspx?s=&Type=RD&YEAR=1950