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.
That also checks out. The game was added to PSN+ just a couple of weeks after the movie came out.
Also, I don't know for sure but I don't think it was on Gamepass, which could potentially lead to more people getting it on PSN+. Could be wrong about that though since I don't actually have Gamepass.