And the very fact of the Function keys' configurability meant that they couldn't be relied upon to always do some specific thing, which kind of let devs off the hook. If nobody's going to try pressing F1 anyway, you may as well not bother to make it do anything. Vicious cycle. (See also, and even worse: the "What is This?" window caption-button in Windows, that invoked a modal "Context Help" mode. It was so non-discoverable — and optional! — that I don't know of a single third-party that ever bothered enabling their software to use it.)

On the other hand, if there's a key on the user's keyboard explicitly labelled "Help", then as a developer you'll feel somewhat obligated to put some informative documentation into your app that appears when "Help" is pressed. "Burning" a help function into the hardware, takes away the developer's choice in whether they want to offer help or not, without needing to impose any sort of centralized QA process. The QA process is something the dev will be incentivized to do on their own, when faced with the realization of the inevitability of users' frustrated expectations when they think pressing "Help" will get them some help. :)