Existing Machinery.
Adding a few hundred transistors for multiplexing is approximately free compared to buying production machinery for several millions - before even the fist chip can be made.
Creating a new chip (family) is for sure a risky bet on the future and takes some investment. Keeping this investment down to a minimum reduces the risk taken.
Ordering, building, integration and ramp up of new production equipment is a quite large investment. Requiring this for a new chip might endanger the whole project, as Management might not be inclined to spend that money on some fancy and uncertain new stuff.
So calling for new, larger packages not strictly needed to make a new chip would be a rather stupid move for engineers. Especially not if one can come up with schemes like multiplexing to make it work with the existing production environment.
The driver for DIP 40 package weren't CPUs, but rather memories (as they left the DIP16 early on) and I/O Chips. With the need for devices like 8155 or 6820 as whole families (and the proven success of early CPUs) new machinery for DIP 40 could be justified.
