Because pretty much all of the major cable channels are based in NYC or elsewhere on the East Coast. A better question though is why can't you get a DVR?
I have a DVR. That doesn't mean I don't want to turn on the TV after dinner, and just see "what's on". 8 PM is prime time, that's when I am more or less done with dinner, cleaned up, checked my email, etc.... And it would be really nice, especially on movie channels without commercials such as HBO and the likes, to then just turn on the movie and watch it. Because at 5 PM, when it is 8 PM EST, most of us west-coasties are still in traffic trying to get home. And of course I hear you make the argument "well, why can't you remote schedule the movie from your work or smart phone", but it just doesn't work that way.
Besides, believe it or not: SateliteRacer said that the DVR penetration is about 50% of customers, so that leaves the other 50% without one. Should they all come home at 5 PM so they don't miss out on their HBO prime time movie?
The world does not revolve around the east coast, even though there are more people living there. The 50 million living in the PST time zone are very happy with our PST prime time at 8. Not at 5.
I bet for the same reason you won't want to have your prime time movie moved to 11, (or 8 PM PST) so that you are still watching it at 1 AM, or have to record it so you can watch it next day. No, just like us west coasties, you would probably want the movie to come on at 8 pm, DVR or not.
A lot of cable channels start showing the repeats at 11 PM EST, so that is 8 PM PST, and I am fine with that.
But don't take away my HBO west coast feeds.
As far as kids channels go: That has to do more with kid bed times, its easier to plan when the kids programs are on more or less on the same time across the country.