This topic is talking about only the tuner TV side of HTPC, so my comments are meant only as far as that subject.
In that context, I think it boils down to lack of awareness and difficulty of set up.
I consider myself to be fairly PC savy. However, the average consumer is not. Even so, I preferred using the cable cos DVR until the monthly charges got to be more than I was willing to pay.
Why? Because it was too easy. And I believe the average person thinks the same way. It makes no difference what magical capabilities a HTPC is capable of (not to the average Joe, anyway). They don't want to set up something that is going to take much effort. And if you want something to really catch on, it has to be a no brainer.
Look at the iPhone. Very overpriced for what it does (IMO), yet it's maybe the most popular single model of smart phone. Brand recognition and ease of use.
Radio was easy. Just turn it on. And radio caught on. TV was easy. Internet didn't start to catch on until it became basically plug and play. TIVO was plug and play, basically (b4 cable cards).
To my knowledge, nobody makes a plug and play HTPC that Joe consumer can simply buy and plug in (if they do, they must be too small to get their name out to the masses). Even if they did, the consumer still has to deal with cable cards, and getting their PC connected to their TV.
Cable cards can still be a mess, even today. My belief with TIVO is that when the average person buys one, they have no idea what a cable card is. If there are issues with that, it's after the fact. TIVO has brand recognition.
If I remember, MS came out with WMC before cable cards, and they never really pushed it. They adapted it for cable cards, but by then TIVO was almost coke, and you still had to get your own tuner, set it up, and get the thing connected to your TV.
Even after setting up several WMC PC's, a server, and Media Browser 3, even I still have no idea what sickbeard is. But with the price of cable continuing to go up, the fact that they still require you to subscribe to 60 or 80 channels you don't want for the privilege of subscribing to HBO, and my cable co finally starting to "protect" many cable channels, I will likely be looking into it.
Sent from my generic not my computer device.