The simple answer is that they see a future in 3D. Perhaps they are wrong. But they still see some reason to invest the money needed to have 3D in their sets.
I wouldn't say it is so much of "a future" in the way most consumers see it (as in I'll be using 3D for years and wondering why I ever bothered with that old 2D stuff) but rather, they can get enough money out if it over some period of time to recoup their investment plus a good/decent profit margin. This can be true if there is "a future" or if it is just a fad or a check-off item that people will pay enough for over whatever time it lasts.
The CE field is very competitive and the usual model is to build the best product that you can this year, then ignore it and build whatever you think is the best product next year... repeat. Whether 3D takes off at a consumer level or not, by the time this is known, the TV makers will be introducing some other new thing to differentiate themselves from the boring old commodity-priced stuff that everyone already has.
Edited by Orion9, 23 June 2013 - 10:08 AM.