For code it also seems most of the text reside on the left. Opening an extra wide window to display text for the occasional long line seems a waste of screen space.
Code is different in several ways. Firstly, code is generally shown in monospace typeface. Secondly, breaking lines too early can be even worse for code than for prose. Thirdly, even with a long line limit, code will generally have lots of short lines anyway, and this helps the eye find the next line when reading. The long lines (as long as they aren't all long) stick out from the rest of the code text, making it easier to read.