I believe the reason people hate on the PS3 is for many reasons.
1) It's has the lowest sales right now. It may be selling better than the PS1/PS2 did in the same timeframe, but it's still selling the worst.
2) The price is high and out of reach for the average customer. And there aren't many games to justify the purchase itself.
3) Sony promised us the world with the PS3, and thus far, has only given us a small country. This may change with time, but most people aren't patient (lucky for me, I am).
4) It's the latest trend to hate on Sony, so the rest of the sheep follow suit.
5) Then there are no doubt some that fear that the PS3 may eventually overtake their precious system (sales and market share), and for some odd reason, it would break their heart to see that. So they just put down everything Sony does to make themselves feel more secure.
6) Then there's the fear of cool things coming out for the PS3 (Home, LBP, and MGS4 to name a few), and they want them, and if they can't have them (multiplatform), they want them to fail.
7) Then there's just @ssholes out there that just want to see Sony fail just for the hell of it. They aren't scared, they just want to rub it in Sony fan's faces.
Well, that's my take on why people keep hating on the PS3 and Sony. I might be right, I might be wrong. All I know is that I typed alot of stuff and it makes me look smart.