They needed private IP ranges that wouldn't conflict with the real internet. 192.168 was just sitting there unused, so they grabbed it along with 10.x.x.x and 172.16-31.x.x.
This is a fuzzy recollection of something I believe I read, which might well be inaccurate, and for which I can find no corroboration. I mention it solely because it might spark memories from someone who actually knows:
Offering an alternative hypothesis seems reasonable given the content of the post.