PowerMax canisters actually have more propane in them than other canisters. And it is the propane that "boils" at such a low temperature.
As mentioned above, I suspect the reason the cans are different is more of a marketing thing for Coleman than anything else. And there is no reason for existing canister manufactures to switch to a new style as the startup cost for a new can material would be prohibitive. And possibly, an AL can in the shape of the current canisters would not be strong enough to support a stove and a full 2-4qt pot. Whereas the PowerMax canisters are not required to support anything.