[coliveira]

Well, I imagine at the time they had some LISP implementations that were very well tuned for specific high end machines, which essentially duplicated Fortran functionality. This is difficult to do for general purpose Lisps like SBCL. It was also probably very expensive.

[pjmlp]

What is difficult is having Apple, Google, IBM, Microsoft, Intel, NVidia, AMD,.... compiler teams budget.

[codr7]

As well as high end machines built for Lisp.