That is a lot of glass - and the expense to produce big tubes gets exponentially higher as the set gets bigger. Some RPTVs are actually cheaper than tube TVs that are smaller than they are in size, depending on the complexity of the tube. These are flat screens filled with liquid where each pixel on the screen is individually lit to produce a display. LCD displays are used also for television screens, and some larger flat screen displays are referred to as plasma displays, because they are filled with a conductive plasma instead of liquid crystal. A higher definition screen has a lower production yield because there are more pixels that can go bad.
Lower yields cause higher prices.
As production methods get better prices will continue to fall, but right now a high definition flat screen display is much more expensive than an equivalent CRT screen. There are ways of using LCDs instead of CRTs in projectors, and both LCD-based FPTVs and RPTVs exist. As you can imagine, projecting a small LCD panel to make it larger is much less expensive than producing one large flat panel. Not many RPTVs are made now using LCDs, but that percentage will increase as LCD technology gets better and cheaper. Now that you know the terms of the digital video world, you are probably ready to ask more advanced questions.