Because the sound on modern ultra thin LCD TV's is crap (there's no room for a decent couple of speakers in the casing!) and a good 5.1 or 7.1 is miles better than anything from an old CRT TV and if you are careful matching your components works seamlessly.
If you want a simple to use good sounding system pair an LG TV of your choice to a LG BB5521A 4.1ch Premium 3D Blu-ray Sound Bar - sound is very good and it is a doodle to set up & use & everything you need in one box + wireless sub.
Bought one of these for my sister a while ago when they were a bargain @ £399 (from Electro Centre) - bit more expensive now but miles cheaper here than anywhere else.
LG BB5521A 4.1ch Premium 3D Blu-ray Sound Bar
You don't even need to go for a smart TV as the soundbar has all the DNLA Smart TV features built in and is wireless out of the box - so minimum of wires - can get by with just one HDMI lead.