Read that Plasma HDTVs have better viewing angle, better or equivalent quality of Video Motion and Picture contrast compared to LED-LCD tvs. I would like to know why the popularity of Plasma TVs is going down and LED-LCD going up?

Burn in is still a possibility and plasma tv's tend to generate quite a bit of heat (Go touch one in a store after it's been on for a few hours, it's hot).  Most plasma tv's also have an airtight glass front panel.  The glass can be a pain in the rear in a room with good lighting (Most living rooms at certain times of day) as glass gives you tons of glare.  Heat and electronics aren't great together, so plasma's tend to have more maintenance issues then LCD/LED. Not a horrible tv in the right circumstances, but in my experience when people add the pros and cons of plasma versus led.  Even with the cost difference, they tend to go led.

There is one more issue: burn in.  Newer plasmas are getting better, but the first 200-500 hrs are very critical. If you don't play games, plasma is the way to go.