It cost too much to operate and it was a gas guzzler. And nobody really needed to get across the Atlantic in half the time at three times the cost, at least, not enough to make it pay for itself. Beautiful technology, but it made no business sense. The speed-cost tradeoff quickly reached a point of diminishing returns. There may be scientific or military applications that will make this kind of tech worthwhile, but nobody out to make a buck is really interested.

Like a lot of technology, it was a toy for engineers with too much government money to burn, primarily made for purposes of national prestige and status for the rich. What commercial aviation really needs is not more speed, but better fuel economy, larger cargo capacity, lower maintenance/operating costs, and the ability to operate off shorter runways. And while they're at it, they could work on the traffic jams and security hassles at the airports. When you have to show up two hours early for a one hour flight, faster planes are not the answer.

Even for military purposes, ever-higher speeds don't necessarily lead to a payoff. The 1960s B-58 bomber still holds several speed records, but they still didn't keep it from becoming obsolete as a weapons system for a variety of other reasons. OTOH, the venerable 1950s low-tech, subsonic B-52 is still an important part of our defense capability.