Existing Machinery.
Adding like a few hundred transistors for multiplexing is comparable free 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. Requireing this for a new chip might mean to 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.
Driver for DIP 40 wheren'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.
