This picture answers it very easily:
To generate maximum longitudinal force you need, in that specific case, around 5% slip. That would mean that the tires has to go 5% slower than the car's actual speed. The only way for a tire to generate force is with slip, hence it goes through (0,0) in the graph.
The real question would be why does the curve has that shape. If I'm don't remember wrong, in the book tires suspension and handling there's a simple tire model. It look like lots of springs in the contact patch that stretch. So maximum force would be when all those imaginary springs are stretched - in a slipping condition. In that model the curve would remain flat and never drop. Dixon mentions dropping due to tire over-heating among others which I really can't remember.
Maybe JT can drop some light on to some of the reasons why tire lose force when fully locked.
) was that when you locked the tyres - the contact patch, relative to the road - had dynamic friction; and when the tyre is still rolling; the contact patch was having static friction to the road - and static friction is always greater IIRC.
