Why hasn't Tesla accelerated Semi production?

“…that's part of the reason why we have not, for example, really accelerated production of the Tesla semi because it does use a lot of cells. And unless we've got a lot of battery cells available, then say, like accelerating production of the Tesla semi would then necessarily mean making fewer Model 3 or Model Y cars.”

— Elon Musk, 29 January 2020 · Watch: Tesla Q4 and full year 2019 earnings call (2020) · 39:22 ↗