The point is how many sales have been lost by not being on steam. Outside of UK and previous Elite players of yore how many Know Elite and Frontier as a brand and can find the website?
Selling via Steam meant giving up 30% of their revenue to Valve who do exactly nothing for that free money. Exposure would be nice but is it 30% nice? That is a gamble they chose not to take.
There is also the fact that they have to contend with Steam's TOS which essentially says they can take your games away at any time for any reason and FD would have no real power to prevent that. Steam can change their TOS at any time, insert whatever wacky clause they like and your only choice is to agree or lose your entire game library. I can understand FD not wanting to be bound by a TOS agreement they have no power over and that can change to literally anything without them having any say in it.