Wow, that didn't answer your question at all did it?
ANSWER: BECAUSE THEY CAN CHARGE MORE!
The hype on selling 'true-HD' to the masses is out of control and despite people claiming incredible results with 90" front projectors that are 480p (InFocus SP4805 & Optoma H31) the reality is that consumers are 100% convinced that a 50" display must be at least 1280x720 (more likely 1366x768) to be good.
BS! You own a 480p set, you know how awesome it is. Paying a grand more for a 50" set makes sense to me. Paying 2 grand more because I pick up some additional pixels is just them raping me for cash as I will be sitting 15 feet away from it in my family room.
Of course, I have a 50" plasma that I got for just over $3K over a year ago, so I don't feel bad for what I paid, but you know and I know that 480p gives phenomenal image quality, and would be a great alternative to people paying more than they should for a 720p plasma that is 50"
I imagine if you put a 480p 50" plasma right next to a 50" HD plasma and calibrate them both you will have people asking the question "Why would I pay a $1,000 more for that?"
$$$