It's part of the different mobile phone standards. Emergency calls get location information with them (AML), can be sent on other networks and without SIM card, they can be dialed on international short codes (eg: you can dial 112 in the US and 999 in the EU and it will connect regardless). There are many more differences.
Using different protocols for emergency call predates any caller location information.
The main drivers at the time were network coverage (your SIM might have no coverage, but a network you are not allowed to roam for ordinary calls in has) and congestion (emergency calls can preempt ordinary calls).
It's part of the different mobile phone standards. Emergency calls get location information with them (AML), can be sent on other networks and without SIM card, they can be dialed on international short codes (eg: you can dial 112 in the US and 999 in the EU and it will connect regardless). There are many more differences.