If I have to be at Heathrow 90 min before departure, and it takes me 45 min to drive there, or 90 min on public transport, then at the other end it takes 30 mins for my bag to get on the belt, clear port control, and another 30-60 min to get to the city. I have a good 4 hours of fixed time involved with my flight. If the flight is just 8 hours, making it 7 hours isn't really going to change things, I've still lost the most of the day.
So I'd sooner be able to sleep comfortably (ie space!) or be able to do some work (11 vaio Pro, 22 hours battery).
Then we have the issue that even going supersonic, we can't just upsticks and do it. We have to get out to see. Climbing to FL390 takes a lot of time, descending from it does as well, otherwise people generally find it rather un-appealing. Add in busy hub airport traffic and we've got at least 2 hours that we can't really speed up.
So our 12 hour total journy (4 hours travelling too / waiting at airports, 6 hours of cruise, 2 hours of holding an NDB) speeding up that 6 hours, doesn't have much effect. Hell go nuts, make it take 2 hours, go for mach 3. It is still going to take me, door to door, 8 hours. 8 vs 12 for the cost of that just isn't going to be worth it. Because ultimately I'll have my own work schedule, I'm not rich enough to have a private jet, and the stuff I fly you wouldn't want to be in for more than 4 hours anyway (well the fuel would run out too). So that means I want a regular, frequent service. I don't care that superfastjet leaves everyday at 1pm. I need to be around till 5pm. Obviously the geek in me would take the fast one, but I couldn't do that everytime.
TLDR It isn't just about being cheaper. It is recognising the deminishing impact speed is having on being the deciding factor. We look at door to door time, not cruise performance. We want a frequent schedule, not one speedbird a day.