Today, commercial airliners cruise between 480 and 510 knots. In the 1960s, they cruised at about 525 knots for a Boeing 707. The main reason for the decrease in speed is simply fuel economy. To go faster would eat up more fuel per-passenger-mile. The new ‘high bypass’ jet engines with their large diameter front fans especially eat up fuel.
One big exception to the speed rule was the Concorde. It primarily flew trans-Atlantic passenger routes at twice the speed of sound. They flew successfully from 1976 until 2003 when a treaty between Great Britain and France was signed. The Concorde only served a small high-end market and was extremely limited in their flight paths, due to their supersonic boom.
The issue is that when something travels faster than the speed of sound, a shock wave is created that causes a supersonic boom. The sound is heard as it passes overhead and is really obnoxious. So, flying over the Atlantic Ocean was a good place that wouldn’t bother people.