The State Troopers have the legal mandate to provide police coverage to areas of the state without local/municipal, or borough police forces. The Metropolitan Police Service Area (outlined in AMC 27.30.135) specifically states "those areas north of McHugh Creek" are subject to the jurisdiction of the Anchorage Police Department.

This was done because areas south of there did not want to pay tax to the city for police protection and chose to remain with the Alaska State Troopers. The municipality made it law that police taxes and services could not be forced on those residents unless a vote was held to expand the MPSA and those areas voted to join the MPSA.

It's not overly complex. These areas are effectively under the jurisdiction of AST unless they elect to place themselves under APD. APD's hands are legally tied, and AST is legally obligated to provide police protection to residents of the area APD can't cover.