> It's a fair observation that you could build end-usery UIs on a LispM.
Texas instruments ran the Interface Builder on their Lisp Machine, which was demoed to Steve Jobs and which then was developed into the NeXTstep interface builder.
> I guess a more important factor is that PCs were cheap enough to create a market for friendlier UIs, and LispMs weren't.
The higher-end commercial applications on the Lispm were kind of user friendly. Using something like the font editor or a graphics editor was not difficult. There was a problem that during its lifetime much of the UI landscape was under development - both in look&feel and APIs. There were even UIs on the Lispm that looked similar to Mac apps (like Plexi, a neural network toolkit).
Symbolics' Dynamic Windows GUI had some polish, which came from being used in some applications and it had seen some substantial investment in development. Where this investment was not done, applications lacked polish and remained experiments, demos, sketches, research prototypes, ...
TI also developed a more conventional UI toolkit for their Lispm.
> I guess a more important factor is that PCs were cheap enough to create a market for friendlier UIs, and LispMs weren't.
The market was small - I would guess that around 10000 machines were sold. Additionally the software was not portable to other platforms. CLIM was thought to support that - with some applications using it - but that was bound to commercial Lisp systems, some of them expensive... Generally I think the Lispm impact on other UI systems wasn't that great.