Because 1001 different Schemes exist and the code between them is not portable because the Scheme standard is so tiny. Scheme code is so hopelessly unportable,

Because 1001 different Schemes exist and the code between them is not portable because the Scheme standard is so tiny. Scheme code is so hopelessly unportable,that you can not even talk about "writing Scheme". You are either writing Gauche, or Chicken, or Racket, and once you write it, your code stays in that tiny little ecosystem with not more than 10 different users each.