Well again, it's how they or any other company looks at making special products for people to get at a certain time.
Personally I do feel the same way about other skins, such as Championship Riven, which I was here during season 2 but didn't own Riven at the time and then the skin has since been unavailable. Since then it's only been available if you won the guessing game at Worlds which was won by a single person last year and like 10 people the year before. So Riot took that information vs how many people have been begging them to re-release it and will be doing so later this year.
So they do look at making certain things available again when time permits, it's not like they don't want to, it's just "is it the right time?" kind of thing. With the last remaining limited skins they have come around saying most of them last as a limited edition for about 4 years or so and then they will make their return after that time period is over. Riven for example was released in 2012 and it's now 2016 so her time period has passed and she will be seen again near the end of this year.
For other legacy content though, they come back at least once or twice a year, and sometimes in Bundles (sure you don't want the rest of whats in there but it's a way to get what you do)