Let's look at Wikipedia for possible liquid rocket fuels. The key item is the Exhaust Velocity. Let's remove any that could be toxic, so no Beryllium, Florine, or Boron, despite the fact that each could make a very efficient rocket.
- $H_2{\space}LOX$- 3816
- $CH_4{\space}LOX$- 3034
- $C_2H_6{\space}LOX$- 3006
- $C_2H_4{\space}LOX$- 3053
- $\textrm{RP1}{\space}LOX$- 2941
So Methane, Ethane, and Ethylene all perform similar to RP1, and in fact slightly better. None compares, however, to Hydrogen/Oxygen. Still, liquid hydrogen has issues that make it difficult to work with. So, what are some of the side effects of these various fuels?
- $H_2{\space}LOX$- Difficult to work with, requires larger tanks than traditional fuel. Requires extreme cold.
- $CH_4{\space}LOX$- Burns clean. Is less energy dense than other fuels, requiring a larger tank.
- $C_2H_6{\space}LOX$ / $C_2H_4{\space}LOX$ / $\textrm{RP1}{\space}LOX$ Basically these gradually become more energy dense, burn less clean, etc.
Burning clean is important for re-usability, no need to clean the engine periodically. That is probably the major reason why it would be invested for Earth based technologies. They also could be looking for it as a path to Mars, as SpaceX has been.
Furthermore, Elon Musk broke out SpaceX's decision to use Methane.
Of these, only the last doesn't apply. Methane is much cheaper than RP1, hence, if you have a vehicle where most of the cost is the fuel, then Methane makes more sense.