Timewalking should always be up

Title says it all, what's the reason behind having timewalking every once in a while? If it was up every week I'm pretty sure a lot of people would be interested in running the dungeons. Some might say because there are mounts involved, well all of the mounts obtainable through Timewalking (both with tokens and the one that drops) are rare and require a good amount of farming before we can even see one dropping.​EDIT for clarification: we could have them rotating so we don't get all TWs at once but 1 timewalking expansion should always be up