Because there's a decision-making process to be made, which requires information to be gathered and considered. It's not like a car where you pull over into a convenient layby and get out to call for a tow-truck.
If you evacuate the aircraft, passengers will be injured, whether just by going down the slides / hit by airport vehicles / hit by hand luggage thrown by other passengers / cold / hot / wet environment etc etc. Even with very serious problems, quite often the safest place to be is on board the aircraft. Once the evacuation is initiated, it cannot be stopped.
Secondly, the evacuation command is not given until the very last line of the evacuation checklist, which also takes some time to run as it has to be done methodically, and likely requires another pause and considered decision before actioning the evacuation signal at the end.