With a LIFO stack of waiting requests, an old request may remain at the bottom while a new request arrives before each completion and is served next. Every service can finish in finite time while the old request waits forever. A FIFO queue prevents this overtaking within one priority class. Nonpreemptive strict-priority arbitration can separately starve a lower-priority class; using a LIFO stack adds potential starvation even among requests of equal priority.
New to topics? Read the docs here!