Lacking infix syntax out of the box is a problem, though not hard to add as a sibling mentioned; for those curious I like linking to this gem dating back to '93 since it's part of quicklisp now https://github.com/rigetticomputing/cmu-infix The rest of the syntax being somewhat alien-ish is also a problem, but it goes away with practice. (https://www.thejach.com/imgs/lisp_parens.png)

But I don't think syntax alone is enough to explain Lisp's apparent languishing (though only apparent since it seems to be on the rise again), because we have Clojure. Clojure has the same syntax issues as Lisp (ok a bit fewer because of no reader macros and there's an argument that bracket variety is more pleasant to read) yet it's very popular all things considered. And because Clojure has a BDFL, if lacking infix OOTB was really an issue (I don't think it's come up in any of the annual surveys), RH could just include one of the libraries for it out of the box. SBCL could do that for Lisp, but it would be non-standard.

To me there are several stronger drivers for language adoption than anything the language itself offers. Here are three: having a BDFL (Python, Perl, PHP, Clojure... not Lisp), having big corporate backing (Swift, Objective C, C#, Go, arguably Java but driven by more than 1 megacorp... not Lisp), being the lingua franca of a very widespread platform (Javascript (web), PHP (shared web hosts), Bash, C/C++ (unix, windows), Java (android)... not Lisp, maybe if the Lisp Machine had won but it didn't).