This trend has been somewhat slowly growing within the past few years. It might be temporary, but given the fact japanese market is heavily oversaturated at the moment, it's propably even harder to produce high quality vn's which at least cover the production expenses and bring in some profit to the companies. This, coupled with the everchanging trends might make a lot of devs shudder, especially when it's often just a single flop that might bring an entire studio crumbling down.
I'm not surprised the quality is dropping in overall. A lot of studios simply revert to fail-safe practices, milking their franchise/flagship games as much as possible, since it's usually produces enough revenue to survive.