@Meddler Clarity for new AP Jungle Enchantment

I was hoping we could, as a community, get a bit of insight into the team's thinking behind this jungle enchantment.

Magus was a while ago, but it was strong on almost all AP junglers as a pure stat stick. When it got taken out in favor of Rune Glaive, some junglers excelled with the new items and others fell by the wayside. I never really saw a detailed explanation for the reasoning behind it, but it was overall a good change in my opinion. Now Rune Glaive, which has itself fallen by the wayside perhaps due to the increased cost of AP in general (or perhaps because many of its star champions have since been nerfed or fallen out of popularity), it is being replaced with Runic Echoes.

Runic Echoes sounds pretty great, to be honest. Movement speed as a jungler is huge. My question is why the replacement again? Why do the AP items keep getting dropped in favor of a new one? If it's merely to promote diversity, why not just keep both?

It just feels a bit like Riot intentionally impacting the jungle meta, but I am aware that isn't your standard operating procedure. So what's the deal? Why do the AP jg items get switched out every now and again rather than just tweaks to the old ones or extra items added?