Earlier accessibility of a top Scheme implementation could have helped make it more popular earlier. Just like with Smalltalk and Common Lisp before which also got into trouble because of good implementations being out of reach of most people.

What about SCM, T, Larceny, Bigloo, Gambit, Chicken, Guile, and Racket? All were open source before Chez. T, in particular, while no longer active, was at the time a pioneer of fast and effective scheme compilation.

One could make a number of arguments about at least half of these. Racket/PLT has been free for a long time but ten years ago, a friend of mine simply left it for Common Lisp for the sole reason of speed. Gambit is great - I'd say almost as good as Chez - but there's no proper modules or native multi-threading. The Scheme->C model is also cumbersome; reintroduction of a native compiler was promised but if I'm not mistaken, still hasn't happened (although some other interesting things did - the recent mobile code support is very interesting!). T was a research vehicle but apparently the closest thing to an accessible environment for it was A/UX on Mac. Say what you want about IBM PCs but they won.