Why APRS rides on the AX.25 packet radio link layer, the same protocol used for conventional packet. APRS sends position and telemetry data in unconnected (UI, unnumbered information) AX.25 frames, so any station in range can hear the beacon without establishing a connection. On 2 meters this is typically 1200 baud AFSK on 144.390 MHz in the US.
Watch out PACTOR and AMTOR are HF ARQ modes built for point-to-point message handling, not one-to-many beaconing, and QAM is a modulation scheme rather than a protocol.
APRS = AX.25 broadcast beacons. Both start with A and the packet lineage is the link.