Methane is easier to reduce than CO2. CO2 is the inevitable product of combusting any carbon based molecule, such as fossil fuels or organics. Methane is basically what happens when you take an organic molecule, and fail to burn it all the way. If you take your stack exhaust and expose it to more heat and enough oxygen, the methane will react to form water and CO2. For every methane molecule you combust to CO2, you will reduce its greenhouse effect to one twentieth of what it would have been, at least for its first couple decades in the atmosphere. So it is low hanging fruit with a big impact.

CO2 emissions are harder to reduce, because in order to get rid of those you need to stop fucking burning carbon, and companies don't want to do that. So the only way for a society to reduce carbon emissions is to transition to a model where capital does not control the economy. Whereas you can reduce methane emissions and still have capitalism if you just apply some gentle economic pressures.