Well, at this point it's far too late. The PS3 launched over five years ago now (wow), and for all of that time, it has offered a free online service. Suddenly beginning to charge for this service would be a thousand times worse than the removal of OtherOS, because it's a feature that pretty much everyone uses.
As for why they didn't do it at launch, the Xbox 360 and its business model weren't enormously successful when the PS3 came out. Moreover, since the PS3 was so much more expensive than the 360, it would be nearly impossible to justify such a monthly charge. That whole "you save money in the long run because of online" argument wouldn't have worked. I know quite a few people who were on the fence, and went for a PS3 because of the free online, and many more who already owned a 360, and picked up a PS3 for the same reason. I do think Sony has benefitted from having a free online service.
EDIT: Also, for much of its existence, PSN didn't really hold a candle to Xbox Live, and if they both cost even nearly the same amount, XBL would have been better value.
(Former) Lead Moderator and (Eternal) VGC Detective 
