They're cheap, simple (don't require a complex BMS), predictable/safe, have a long-enough lifetime and are easy to recycle. The biggest drawback really is weight - and thereby shipping cost - but for long term stationary applications that doesn't matter.
Lithium Ion attached to a supercapacitor with a sophisticated BMS is a much more complex setup-- yet even so, it will probably end up being the gold standard in the long run at this point, I would say even in most places lead-acid is still being used. I would not have predicted anything like that even five years ago, but barring some unlikely breakthrough it seems like where we're headed.
Honestly it’s just market inefficiencies at this point. I build battery packs for drones and RC planes and the lead acid stuff is just garbage. They can’t perform to more than 60% of their rated capacity and then start voltage sagging. The energy density is pitiful. A small UPS under your desk with LiFePo4 cells would outperform the lead acid 3-4x for the same space and last longer for the same or less money.
Lithium Ion attached to a supercapacitor with a sophisticated BMS is a much more complex setup-- yet even so, it will probably end up being the gold standard in the long run at this point, I would say even in most places lead-acid is still being used. I would not have predicted anything like that even five years ago, but barring some unlikely breakthrough it seems like where we're headed.