It’s for a very simple reason, or reasons.
1) LED’s are super cheap. I can get a bag of 100 3mm LED’s for $1.95 – the effective cost per LED is 2 cents each. If you are an electronics geek hit Marlin P. Jones Associates, Inc.. They’ve got some good deals on stuff.
2) Current LCD TV’s use fluorescent tubes which require AC current. This means inverters need to be on most TV’s. This is expensive.
With LED’s all your really need to do is limit the current a little bit, something easily accomplished with resistors which are also cheap passive devices. No transformers, no oscillators, just LED/Resistor combination.
So that’s why we’re seeing more LED TV’s come out. They’re less expensive to make.