It was almost mainstream in the 1980s when there were four Lisp Machine companies (Xerox, Symbolics, Texas Instruments, LMI) and symbolic AI, particularly expert systems, were used. Not so much now.

I still use it. If you know it you can be much more expressive and productive in it than in other mainstream languages, and when there's something you need that's missing from Lisp, you can extend Lisp to include it.

There are reasons it's fallen from favour, some of which lisper has given here. It's a mistake to languages necessarily succeed or fail on their merits.

In the late early 80s it was rather popular but it required expensive specialized hardware and software. Or you could execute it on more common hardware, but memory requirements were pretty high, which adds to the expense.

It's not like in 2017 where you can have many excellent Lisp implementations for free and they run blazingly fast on your machine, without needing more memory than what you have already.

It also has something to do with the triumph of UNIX over the Lisp machines