There are definitely more scientists so it could be that the number of researchers has gone up faster than the number of areas of exploration and you’re just seeing the effect of more researchers per area. This saturation would be exacerbated the number of areas for cutting edge patentable research has gone down.
The other two possibilities are cultural. There may be more of a cultural equilibrium of patenting everything that seems vaguely valuable as soon as possible instead of waiting for a higher value patent or group of patents because you know that everybody else will do the same thing. This may be exacerbated by both Bayh-Dole but also the cultural expectation of how valuable science should be. This cultural expectation didn’t really come about until after WWII and perhaps took years to saturate the system. The other cultural explanation for the missing slack might be conservatism around areas of exploration. If there are more incentives for groups to do research on known good ideas then naturally there will be fewer discoveries that nobody else was working on.