My guess is that because the accent was put on serial computations. I may be wrong but for for me the big selling point of a(n ideal) lisp machine is that if made right you could cut the machine in half and get two lisp machines. Conversely, two lisp machines working together are a bigger lisp machine, while two Turing machines working together are not a Turing machine.
Maybe now is the time for these machines, or better machine parts randomly assembled by the big blind watchmaker [0] over the web.