It is mostly about the developers decision and the main publisher descion to come in an agreement... here is matrix Code force already have a publisher, in which is Matrix Games, to use another publisher like steam or any other 3rd party is like pushing it for some. And probably Erik or the team sees not much advantage in it, except the only thing an increment in sales but still they have to pay to steam or impulse or whatever company to put them in their database to sell. And against what they do and what matrix have and do, as matrix you can buy, download, patch it when ever you want, download it again if you lost the game, and no software pushing your buttons.
Now that you mention Impulse, i do prefer that one than any other, you have the freedom to use it when ever you want.
*That* is the exploration that awaits you. Not mapping stars and studying nebulae, but charting the unknown possibilities of existence.