Sonic Forces and Shadow of the Colossus were both recent additions. I believe they were offered the same month in the midst of the worldwide lockdown. People were home and PS+ activity was probably way up so people paid attention to the free games on offer. Also Sonic recently starred in a hit movie.
http://www.youtube.com/watch?v=F1gWECYYOSo
Please Watch/Share this video so it gets shown in Hollywood.