It makes sense that Lisp would run well on Lisp machines. I don't know why Symbolics didn't succeed more than it did. Alan Kay has complained that the hardware architecture that started in the 1970s and became dominant was deficient. It might've become popular just because it was cheaper. It didn't run Lisp or Smalltalk well at all, and so they were "killed", to use Kay's term. The languages people became familiar with were driven partly by what became this dominant computing platform; languages like BASIC, Pascal, and C, and all of their follow-ons. This platform grew way faster than the Lisp machines did. Unix essentially rode that wave with the "worse is better" approach. In comparison I'm sure the Lisp machines were considered proprietary, and "weird" or obscure.
What probably also did damage to Lisp's popularity was a perception problem. I talked about this in another comment in this discussion. In the schools Lisp was taught almost exclusively as an AI language. Scheme was different. I remember when I was in college the engineering school taught their students Scheme, and had them do all of their programming assignments in it. But within the CS schools Lisp was seen almost strictly as an AI language. I think this limits its appeal. If students took AI, they learned more about it, if they specialized in something else they learned Fortran, Pascal, and maybe C. Thats was it. AI was such a specialized and problematic field to begin with as well. It seems that hardly anyone saw that Lisp was just as good of a general computing system as C and Unix is on traditional processors.
Smalltalk is the same way. I remember I was taught it (briefly) as a programming language, even though it started out as the operating system of the machine it ran on. It has a rich history, yet most people who have even heard of it think of it as just a language that runs on a VM, which is actually a screwed up way of looking at it.
Another aspect to it is Lisp represents a very different view of programming. Most kids/students are taught imperative languages as their first language. This is encouraged all through school. By the time they meet Lisp it's a bridge too far. That's how it was for me years ago when I first encountered it at school. I had a terrible teacher, so I got no help there, and Lisp was so alien it made my head hurt. I couldn't figure it out. It wasn't until I found a good teacher, and then some good reading material on it in the years that followed that I finally began to grok it a little.