Official reason: it’s a metaphor. If your raw, unstable thoughts are lava, then Obsidian is… well, Obsidian. It is your crystallized (and beautiful) knowledge.
Other reasons: we like Minecraft and Obsidian is (as Licat told me) the hardest block in the original Minecraft (he started playing during alpha I think). Also Obsidian sounds cool to me, personally.
Coincidentally, Fallout and the Outer Worlds, some of Licat’s favorite games, are related to the game studio called Obsidian Entertainment. I say “related” rather than “developed by” because most of the better Fallout games are developed by Bethesda (and Black Isle), not Obsidian.
Edit: I thought about putting the above explanation in FAQ, it’s not actually that frequently asked 