I think the reasons for doing less races are quite simple. Firstly, we have to adapt to the real F1 calendar, if you had a look in the the FSR calendar and F1 calendar, you will never have F1 and FSR in same weekend, personally I think its a good point for the calendar.
Secondly, there are some free spaces, but some people maybe doesnt want two races in two weeks (also for the marshalls have a bit more time to post the penalties, so its a bit easier to post before the next race), I understand that they have more things to do than just Sim-Racing.
Finally, the number of drivers in the latest races came down too much, 15 drivers in the last race of WS is a very low number.