1. Overall market for exotics is slumping
2. Mac builds more cars than there are buyers
3. x-factor: 720S is premium-priced
With regards to #3, I think many of us forget that the 720S, spec for spec, was priced at a massive premium to the true previous-gen car, the MP4-12C. Of course we are several years on, and one could also argue that the sport series took over that price bracket. Nonetheless, the 720 is kind of more Aventador-priced. Many early MP4 owners on this board thought the MP4 was priced high, relative to Mac's startup position in the market. I never thought so, but I remember it was a point of contention among early adopters.
So now that we are in a bit of a perfect storm (i.e. all 3 factors listed, above), naturally the haircut is going to be big.
I will say the 720, styling-wise, makes a lot of other exotics look outdated, pretty much all other McLarens included. I saw a couple up close this morning and was surprised how much the design has grown on me. Was it the last car with Frank S. as director?