This car really does make very little sense, and the little sense it does make, it just makes it look weak.
The only reason I can think of for not just making a lighter and more powerful version of the 12C - which is something Mclaren has already showed they can do quite well - and instead introducing a whole new model is, that the intended engineering changes, like for example implementing the dynamic ride height system (which would probably give it a big edge against the competitors around a track), must have been too extensive. Combine that with the popular sentiment that the 12C looks too bland (therefore perhaps creating a need for a brand unique look) and Mclaren's probable satisfaction with how the front of the P1 looks, I can imagine how creating a whole new model might have seemed like a good idea.
Of course it isn't, but then again, Mclaren has a habit of not registering that maybe even something else than just speed and things you can measure might make a car attractive, so it wouldn't surprise me if concerns like their car being just a mash of two already existing designs - that were already accused of being overly similar -, like their car not being unique because of limited time and funds - and so in turn looking cheap, might go unnoticed.