Viruses evolve, some quite quickly. The flu isn't the fastest (looking at you, HIV), but it's up there. Over time, existing vaccines train your body to fight something that doesn't quite match what's in the wild (i.e. efficacy goes down with time). That's why there's a different seasonal flu vaccine every year.
They create flu vaccines on a yearly cycle, and a pandemic can kick off in a matter of weeks and months, so if it doesn't match the preplanned cycle, they'll have to invest more resources to creating the most up to date vaccine off-cycle.