I can't find a citation, but the rationale I've heard is to make it easier to learn how to use a Macintosh (or a Lisa) by watching someone else use one.
I did dial-up tech support in 1999-2000. Lots of general consumers who'd just got on this "internet" thing and had no idea what they were doing. It was SO HARD to explain right-clicking to them. Steve Jobs was right: more than one mouse button confuses people.
What happened, however, is that Mosaic and Netscape were written for X11 and then for Windows. So the Web pretty much required a second mouse button. Eventually Apple gave up and went with it.
(The important thing about computers is that they are still stupid, too hard to use and don't work. I speak as a professional here.)
Here's the new thread for posting quotes, with the usual rules: