Materials will not be buyable with credits because the current methods for obtaining them are precisely the reason they even exist. They are a way of forcing you to engage with gameplay features that you would otherwise ignore.
The issue, obviously, is that you would otherwise ignore those features because they're boring tedious trash, which is why people instead instance-flip on Dav's Hope/Jameson's Cobra/Crashed Condas rather than get that stuff "properly". To allow you to buy the materials would mean giving up on forcing your face against features that you would much rather skip altogether. It would mean accepting that a lot of the things they designed just plainly do not work and people don't want to do them. And so far they're stubbornly opposed to doing that bit of soul searching, which is why you still need to occasionally go fetch the Wake Scanner and go on sniffing ship butts so that FDev can continue to pretend that whole mechanic isn't a complete waste of time.
The problem for me is how we get pressured into doing it. If I'm landing into a planet or searching random USS because I want to do it as an exploration thing, instead of because I need a specific material right now, it is not as much of a chore.
I have never mined and I would like to continue that way. Some tech broker stuff can be acquired through a FC market. I would have no problem going into material farming to supply a demand, and if the supply is purely player-driven, it would mean the indended gameplay loop was achieved for every material out there.
This is the opportunity that is being lost here. It is fine to not enjoy some parts of the game, but if they are connected together, you can simply not have to go through the gameplay you do not like.