It's physics. The coils in the base and the devices are magnetically connected (same in your toothbrush, BTW) and as current passes through those coils, it produces heat along with the current to charge the battery. What I have read is that because the device AND the base both warm, one is not bad, but trying for three was a technical challenge that couldn't be overcome in a way satisfactory to Apple. Now add in that, unlike your toothbrush, the devices and the chargers have to communicate and the plethora of close by signals plus the heat from three at once just was not possible to do. Your toothbrush has either NiCad or NiMH batteries in it, which are basically pretty dumb batteries, but work well in that application. The problem is that they have very low power density (not a lot of storage in a big physical package), but for brushing your teeth for a couple of minutes, no big deal. And the chemistry of Nickel batteries is pretty tame. They won't overcharge and even if you push them, they won't do anything strange.
I don't want to repeat the whole article, but here is a good place to read about the "adventures" of Lithium batteries:
Lithium-based Batteries Information – Battery University
and
Safety Concerns with Li-ion Batteries – Battery University
Every lithium battery has an intelligent circuit in it that monitors the charge, temperature and the charging/discharging rate, and controls how the battery performs to try to avoid runaway thermal activity. When charging, that includes telling the charger to reduce the charging current to stay within safety limits. Therefore, each of the wireless devices has to communicate with the charging pad to say, "I'm getting full or warm, slow down." One device on one pad, no problem, the signals are all pretty weak but the battery and charger pad are close together so it works well. But three, in close proximity? I suspect some cross talk happens that was, again, just too hard to figure out in a reasonable time. I don't think this is a Jony Ive issue, but one where Apple expected the engineers could figure out the tech problems, got close enough to make an announcement, betting they could solve them all, but in the end, physics killed them for now. Will someone else solve the issues Apple didn't? Well, I suspect that AirPower-like devices will come out, even if they haven't solved the problems to the level Apple wanted/needed. And those devices will eventually kill batteries, maybe with fires, which we can only hope don't hurt any people.