It could be they are only just getting to your Tesco lil.cutie.pie...I know they were in my local stores for ages in massive quantities but only appeared in another Tesco I go to in the city a couple of months ago!
As StrawberryReef says - only the main 6 minus Fluttershy so far this way - we had these in the UK before we even got blind bags, I remember being a little worried this was the only way we would be getting G4 "Ponyville" size ponies. And I think they were in other parts of Europe really early in 2011 as well.
In the UK Asda never stocked them this way but Tesco have never [yet] carried blind bags - the stores much have taken different options for that price point item.