Probably works similar to the MCU, where you don't have to accept a challenge if you don't want. The only time a challenge is mandatory would be when there's a power vacuum after the ruling BP dies. S'Yan was probably getting up there in age and decided to host a Challenge Day to be relieved of his duties. I imagine T'Chaka did the same thing in the movies.
I do respect that T'Challa is still technically king though, Shuri's just acting as the Regent. Still don't like the divvying up of his traits, but it's fairly obvious that if/when he returns to Wakanda he'll have full authority.