It took a while to get there. One would think of end 80s / early 90s. Remember, there were probably only around 10000 (ten thousand, not ten thousands) Lisp Machines ever produced. A 40 bit Ivory 3 processor from Symbolics was basically slightly faster than a Motorola 68030 processor, but with larger memory capabilites. Memory was expensive on stock hardware, too - but not as expensive as the 48bit wide ECC memory on a Lisp Machine. Add to that a Megapixel screen, a large disk, a tape drive, a faster graphics card,...

There was little point investing money into a hardware market which did not produce cheaper and/or faster machines, given the small market.

There were a lot of interesting applications development on Lisp Machines, but there was no point to deliver them on that expensive hard- and software. Development environments were catching up. So a $50k ART expert system development system was replaced by a low-cost CLIPS on machines with less hardware/software costs. It also moved away from Lisp, as Lisp was extremely unpopular (and with almost no funding left) in the 90s.

Nowadays a Lisp on a M2 processor from Apple is 1000 times fast than on the Lisp Machine from 1990. That's just a single CPU, we are not even talking about GPU or Neural network chips. Expensive 40 MB main memory from then is now 8 GB entry level.