I suspect (based on no evidence) that ‘path of least resistance’ had a fair bit to do with it. Napoleon could reshape Paris because he was totally and utterly in charge. Trying to reshape NO would involve trying to get everybody on board at the same time, when what you’d actually get is years and decades of legal challenges informed by property rights (but fundamentally based on NIMBYism) while in the meantime absolutely no progress was made on anything.
In the abstract, turning NO into a nouveau Venice sounds like a neat idea. But there are half a million people who’d be intimately affected by that and who’re unlikely to go along with it. Hell, look how much trouble the US is having trying to get people to wear masks.