Because...'centralized dispatch'!
No, seriously.
Let's say that you are in Black Diamond, and call for an ambulance. You will speak first with the Calgary 911 PSAP (Public Safety Answer Point) that will ask you where you are calling from, 'cause the Calgary 911 PSAP answers 911 for a significant chunk of Alberta (not just Calgary).
So let's say you tell them "I need an ambulance in Black Diamond". You are then transferred to an AHS PSAP. There are three in Alberta (Calgary, Edmonton and Peace River) and you can get your call sent to any of the three depending on which has a free operator (The 911 PSAP has no control of which of the three AHS PSAP's your call is sent to). If all three are busy (which happens) then you wait until someone's free to talk to you. So despite your call starting in Calgary, you have a chance to be talking to someone in Calgary, Edmonton or Peace River...who will verify your location and send you the 'closest' ambulance.
But wait! You tell the AHS phone operator that you need an ambulance because a riot has broken out at Marv's since they are out of hamburger meat, so there is violence involved. The AHS person will stop the call and transfer you to the RCMP OCC (Operational Communications Center) of which there are two in Alberta, one in Edmonton and one in Red Deer. So now you are talking to the RCMP about this, and they are on the way...you will likely be transferred BACK to the AHS PSAP, which of course means you can get any one in any of the three cities again.
Time starts to really add up.
Then, on top of all of that...the AHS PSAP says that the Black Diamond ambulance is currently being held up in a Calgary hospital for a drop off, and the nearest one to Black Diamond is Jasper (I only wish I was joking...)
So yeah, dispatch is a clusterfata