I know some skins are part of a bundle but i think blizzard should putt all previously sold skins in the shop permanently mainly because some players are newer and have not had a chance to purchase older skins.
You can still put the main new skins and bundles in the shop because they are new but i think there are players who would like to buy older skins i think.
Do you want to attend birthday parties in the past of all your new friends you met in the last year? Sure you might, would prolly love to and even pay to do so. But they are in the past and this is now.
The upside tho, they will have more birthdays (hopefully). You can attend those, and who knows, they might revisit an old birthday party venue/theme/etc and you can enjoy the new one as if it were that old one too.
These events in a person’s life are only celebrated as few and as far between in order to make them special that way. Same exact concept is at work here, and is what is driving you nuts over them not being available now to you. You want them, you cant just have them, not without some time travel hijinks. Given thats not a likely option for you, just be patient and they will circle around eventually. The Murloc skin for shaman required some special stuff to be done that i couldnt do at that time. Now a few years later I just got it for gold.
Stop acting so entitled that all things should be made available to you whenever you want them to be.
It was a suggestion for the new players and people that missed certain deals and once again its just a suggestion.