i'd wager it's 100% a cost issue, the same reason that engines are 1.6 l, ECU is standard, etc.
The new 1.6l V6 are more expensive than the previous generation of 2.4l V8's (which were homologated for 10 years).
Formula 1 is a sport as well as a technology industry and the regulations are constructed for many different reasons - competitiveness, safety, cost, feasability etc.