I think it's a combination of reasons: people aren't used to the syntax, so it turns some of them off from giving Lisp a try, Lisp machines didn't take off and

I think it's a combination of reasons: people aren't used to the syntax, so it turns some of them off from giving Lisp a try, Lisp machines didn't take off and their competition didn't have great Lisp support, when computers became more powerful and better suited for Lisp it was seen as older and not as hot as the new things like Perl and Java, and there's still the perception that because Emacs offers a truly first-class Lisp experience that other editors won't work so well for it. And of course because of those various reasons the Lisp community is rather small so it creates the vicious cycle of 'the community is too small and there are no libraries so I won't use it' into 'there are too few people using it so there's a small community and less manpower to make various libraries'.