Upton talked about this in an interview. It's partly the interplay of manufacturing at this point, too.

When they have parts, they'd like to catch up with the backlog faster. Because the 5 doesn't rely on the robots that place through-hole components, its production can to a degree "overlap" with the Pi 4, basically increasing the total number of boards created per hour.

Having two SoCs that don't require the same machines also helps ease a bottleneck there.

That seems like talk about the future. There can be only one bottleneck at a time.