[thenoblesunfish]

It's because the types of things people write in Fortran (high performance science codes, for example) tend to be monolithic, single-purpose programs. It comes from a time when a code really was basically one compilation unit (and doing that is such a nice simplification that I support it, for science). With code written for the web, shared through package managers, etc. it makes more sense to use the uncountable noun instead of the countable one.

[gpvos]

I've never seen the word used in the plural (for computer programming code) until today, having been in computer science for 35 years. The uncountable form definitely dates from way before the web.