On the other hand, this is explicitly planned as a long-term system. So they have to make sure it works right, because if it doesn't, they'll be stuck with it for years.
There's likely also some under-the-hood stuff that was changed for it which needs testing.
BfA/SL systems didn't need as rigorous testing since they knew even if they didn't work, they'd no longer be an issue in the next expansion.