They were some of the first Legendary skins, and were released prior to the Demonblade Tryndamere release, which set the new quality standards for what it means to be a Legendary skin.
If I recall correctly there was actually a partial refund given to those who owned those skins already, and you'll notice that most of those old Legendary skins are no longer available.
You can still purchase two of the older Legendaries (Gentleman Cho'Gath and Brolaf), but the following are all no longer available for normal purchase:
- Annie in Wonderland
- Red Baron Corki
- Alien Invader Heimerdinger
- Firefighter Tristana
- The Magnificent Twisted Fate
I can't answer for why they weren't brought up to Legendary quality when visual updates happened, but my guess is that the time/effort/cost wasn't worth it in some cases, and the fantasy didn't live up to Legendary standards in others.