Methane is pretty directly usable. Generally it costs some additional processing to produce higher hydrocarbons, which may be worth doing. But a system that mak

Methane is pretty directly usable. Generally it costs some additional processing to produce higher hydrocarbons, which may be worth doing. But a system that makes pure methane can be used as the first part of some other, optional, system to make other hydrocarbons from that. You don't need additional equipment and processing facilities, which is important for Terraform Industries because it wants the sytem to be as cheap as possible. Making methane is the cheapest way to get a transportable, useful output (hydrogen is notoriously difficult to move around, methane is a lot easier, and of course propane etc is a lot easier still).