Right now, several methane-fueled rockets are in a race to orbit. With Starship from SpaceX, Vulcan from United Launch Alliance (ULA), and Neutron from Rocket Lab, all of the most active US launch providers are committed to using methalox-methane and oxygen.
Upcoming launchers such as New Glenn from Blue Origin and the Terran family from Relativity Space are also on the way toward flight, while the Chinese ZhuQue-2 rocket from Landspace may even be a favorite to fly before any of the American vehicles.
The answer to why methane-fueled rockets have not flown before is a matter of chemistry and engineering complexity. But as new designs prioritize reusability as well as in-site resource utilization (ISRU) for missions to Mars, the combination of methane and oxygen has become the standard for next-generation launch vehicles.
Combustion stability is especially problematic in comparison to the two most common liquid propellant combinations: kerolox (kerosene and oxygen) and hydrolox (hydrogen and oxygen). The boiling points of hydrogen and Rocket Propellant-1 (RP-1) kerosene are very different from that of liquid oxygen (LOX). However, the boiling point of methane is very close to its oxidizer.
For a hydrogen engine, the combustion occurs in a state where oxygen droplets are surrounded by hydrogen gas molecules during ignition, and the reverse occurs for RP-1. For methane, the boiling points are similar, which means there is no obvious state in which both molecules will be during vaporization and combustion. This can lead to combustion instability and makes methane harder to work with as rocket fuel.