A linguistic oddity

I just got mail (via one of the NetBSD lists) that went something like

"...behaviour... (NetBSD7 did so, ...). ...other behaviour... (NetBSD7 didn't so, ...)."

This is neither here nor there in itself; it is plausible, based on name and originating domain, that the person who wrote it isn't a native anglophone, and this is exactly the sort of point on which I cut non-native users of English lots of slack. The reason I'm mentioning it here is that it provoked me to think: why is it that I am fine with "did so" but not "didn't so"? (For anyone reading this and wondering what's wrong with the latter: it needs to be "didn't do so".)

Is this just a random idiosyncracy English has acquired, or is there an underlying rule? I have the feeling it's the latter, but I haven't come up with anything that feels right for the rule.