One theory I’ve seen put forward on YouTube is that this is because modern digital cameras use a linear LUT when recording, which means that images displayed to everyone during the production process are naturally very washed out compared to how they would look colour-corrected to sRGB. As a result, production staff have become so used to the washed out colours of the linear colourspace that they’ve become accustomed to it, sub-consciously viewing a true colour-corrected version as being wildly over-saturated. The end result is that the films they’re delivering are desaturated because that seems normal to them!

It’s a plausible story if nothing else, but I would also guess that a large part of the reason is just fashion & trends in film making. A more saturated look will return in time no doubt.