Here is a chart of Japan's yearly box office numbers starting with 2009. Look at the amount of $ Pokemon made in 09 then click the years 10-16 to see the box office numbers decline from 48.7m in 09 to 15.9m in 16.

http://www.boxofficemojo.com/intl/japan/yearly/?yr=2009&p=.htm

I also believe that TPC may have wanted to cut back on the amount of mythical Pokemon.