Making helmets mandatory would probably discourage cycling. That happened in Australia at some point, and nobody wants that to happen here.

That said, some cyclists here seem to be downright suicidal. Helmet or not, if you run a red light against traffic while watching your phone, you don't seem to be very attached to living. I've even seen parents direct their kids into ridiculously dangerous situations. Fortunately car drivers pay attention even to stupid cyclists here.