The usual analysis is that Common Lisp killed the Lisp machine, that is, as much as some people will deny it, Common Lisp was designed with implementation in mind (anything implementable is designed with implementation in mind) and the intention was that it would get better-than-Lisp machine performance on machines like the VAX, 68k, 80386, which soon became the mainstream.

An alternate OS is an appealing idea in many ways today but runs into the problem of "where do you get your userspace?" Make it POSIX compatible and you can run all kinds of C code like the GNU tools and other things you find in a Linux distribution. Make something radical and new and you have to write everything from scratch and so do all your users.

Aren't these points similar to the problems Plan9 tried to solve and the ideas it presented (eg. distributed filesystems)? The key point with Lisp Machines was the specialized hardware which we don't seem to need anymore. But attempts have definitely been made, it's just that it usually ends when you don't have enough vendor support (be it hardware, or software).

How is the architecture of a lisp machine different that "normal" computers. I did the first half of the NAND to Tetris course and thought it was super interesting. Since, I have been thinking about using the HDL from the course to play with other computing ideas (a turing machine, cellular automata, one instruction set computers). I have never really found anything about how a lisp machine would be different in terms of hardware though.