Notes was like a combination of the worst qualities of SharePoint and Excel. Like SharePoint, it was "enterprise software" first and foremost, so no attention whatsoever was given to providing a friendly user interface. And like Excel, while to technical people it looks like a God-spited monstrosity, it allowed a certain type of non-technical person to solve problems on their own that would otherwise have required buying expensive software or getting programming help from their IT department (which, in the enterprise, good luck).
So it ended up growing like kudzu, wedging itself into all sorts of weird niches. And because it had the affection of both the enterprise "IT strategists" and that certain type of non-technical end user, once it grew into one of those niches it was insanely hard to dislodge from it. You had to point your flamethrowers in two directions at once.