Found this link in one of the responses on that page:
https://superuser.com/questions/784978/why-did-the-ietf-spec...
Similarly, I expect Postel picked 192.168 because, at the time he made the
choice, it was the next available, or nearly the next available, network to be
assigned from the former Class C space. This probably can't be proved one way
or the other, but the pace of address assignments shown in the RFCs strongly
suggests that they would have been in this general vicinity around 1993-1994
when the assignments were made. (Addresses in 192.159 were being assigned in
1992. No dates are available for assignments in 192.160-192.167 as these were
at some point reallocated to RIPE.)