I trust that historians and literate people were aware of Pompeii's existance but it seems baffling that the local population would've simply forgotten about the existance of two very important cities in the region.
They weren't exactly small. Pompeii at the time of the eruption had a part of the Roman fleet there. It seems hard to believe the local population simply forgot and it seems more likely to me that they didn't really have a reason to care.
But this was such a cataclysmic event, how do local populations simply decide not to care that two large cities fot wiped off the map in a single day.
They weren't really forgotten entirely, Pliny the Younger recorded the destruction of Pompeii in a letter to Tacitus in great detail including the later monks who would copy out these letters into manuscripts (the reason we can read them today), although they would not known the location of Pompeii itself. So at least in educated circles the destruction would still be known.
As to the inhabitants of the local area, the sheer extent of the ash covering the city would make clearing it all very difficult and rebuilding near impossible. We do find examples of later tunnels dug in certain places before major archaeological excavation began, perhaps residents returning to collect belongings or later roman explorers, either way Pompeii had been looted by the time modern excavation began. Evidently the memory of Pompeii did continue for a long time after. But the main problem here is that we don't truly know how the population felt. The only sources we have for the destruction of Pompeii come from rich and educated men who can afford to be relatively unconcerned. The common people who may not even be able to read would only really have access to this knowledge through stories told to them and over time these stories would become less and less important, slowly turning into almost mythical stories until it was eventually forgotten. So it is entirely possible that for long after there was talk about what happened and the people did indeed care, but as happens even in the modern world disasters are soon forgotten as people get on with their lives, the only difference being the common people in the surrounding area had no way of recording their thoughts and we only see the event though the eyes of the educated elite.