Same reason I wouldn't even think about a Powerwall (or anything like it, not picking on Tesla specifically) inside my house. If it could be in an outdoor enclosure a safe distance away, maybe.
Lithium batteries do not need external oxygen to burn and toxic smoke coming up from your backyard with no way to reach the source doesn't sound like a fun proposition.
Unless you happen to have a coal seam in your yard or live on a peat bog it's overwhelmingly more likely that it'll run out of fuel before it hurts anything.
People have been digging fire pits (or building up the ground to the same effect) for millennia because it makes fires more controllable.
Edit: Apparently they can be installed in some interior spaces. I stand corrected.
https://www.tesla.com/powerwall
"The compact, all-in-one construction features versatile mounting options for indoor or outdoor spaces."