The broadcasters don't want to upgrade yet. Most are still using Mpeg 2. The ATSC 3.0 standard is set to be revised in late 2015, so you probably seem adoption by some broadcasters in 2016.
No, because upgrading means consumers need new hardware as well, and thats always a pain to sell them.
Its still years away, but so is widespread 4K, since its the same problem - needs updated hardware for the consumer to get any benefit.
At this point, I wouldn't worry about 4K capability of your HTPC yet. There is no real content available outside of rare demo material, and while you can spend thousands on a high-end machine that may (or may not) be able to deal with future 4K formats properly, just waiting a year or two until content actually shows up will also allow you to buy much cheaper hardware. If you actually happen to have a 4K display already, a good GPU is all you need to convert the 1080p content to 4K for the display with MC's RO HQ mode, for example. Handling actual 4K content however is another beast entirely.
And don't get sucked in by the "professional" or "business" hardware. A professional GPU isn't actually faster then a consumer GPU, it just has a slightly different feature set. Heck, my consumer GPUs have a DisplayPort if thats whats you are after. Even the Intel integrated GPU on my mainboard has a DisplayPort, and that one basically comes for free.
In short, PCs are not completely ready for playback of all 4K content thats out there. But its getting there. Software is being optimized for it, hardware is getting faster, new codecs are being developed.
Its certainly an exciting time if you like that kind of thing. Even if my own hardware, which is pretty high-end, can't keep up with all the content, its nice to see it grow and get better!