In his heyday, Tortimer had a town to rival the best of the dream towns out there. He picked up trash every day, weeded, invited the best of the villagers to live with him. The town was bursting with PWPs and flowers.
But, like everything, he got old. Unable to do anything other than sit at his desk, PWPs fell apart and had to be taken down. Flowers were trampled or wilted, lost to the cruel glare of the sun. Weeds sprouted, and trash littered the beaches. When all but 5 villagers moved away, Tortimer decided that he needed a new mayor, and called his buddy, Rover. Rover saw you at the last minute, as he didn't REALLY want to be mayor and give up his lifestyle of traveling the world, so he practically pushed you out at a stop that wasn't yours. Blinking, confused, not sue who this 'mayor' was or why everything was so odd, Isabelle decided to let you put your own name on the town and assured you that you were, in fact, mayor, as anyone could be better than Tortimer at this point.
That's part of why Isabelle is always at the Town Hall... She's doing most of your job for you, while you run and play and fish. You just THINK you're important.