I don't think you realize just how fast lead acid batteries deteriorate when using their smaller charge capacities. In cars and many electronics they are used because they can be shamelessly charged to 100%, and they are supposed to maintain 100% or close to it. They require constant monitoring to be stored properly in a way that lasts. Using them up below 50% will decrease their lifespan significantly. Lithium batteries are quite different, they shouldn't be charged to 100% but in turn they can use a greater amount of their charge while holding more of it without significant deterioration. They have significantly longer lifespans when used properly. Before we had gasoline cars we had electric cars that used lead acid batteries, there's a reason they stopped being used.

What China wants to do is eliminate the older lithium ebikes because they were built with barely any safety regulation. To do this, they need to offer a cheaper option to their citizens, and the only way they can essentially do this without the original problem persisting is using lead acid batteries, because even a cheap lead acid battery with a cheap charger isn't going to fail spectacularly like the lithium ones. LiFePO4 are far safer than lithium while still having higher energy densities, but you won't see those get promoted because they would be costlier.