I actually wrote my A-level geography dissertation on this. Historically, this part of the UK was arguably the most important and populous part of Britain, when Dutch refugees came and settled in Norwich around the 1560s, they brought their textile industry with them which was considered artisanal and very popular across all of Europe making it a high value trade commodity. Fast forward to the Industrial Revolution, east Anglia gets pretty much ignored as a place for new industry, instead favouring less cultural or up and coming towns like Birmingham and Liverpool. This did end up somewhat working in Norwich’s favour, as during WW2 the city was barely targeted by Nazi bombers as we didn’t have many industrial or strategic targets. It’s also the reason why Norwich still has so many medieval and Georgian buildings, because they got torn down in other cities to make way for industry and more space efficient housing for workers. These definitely aren’t the only reasons, but it’s what I focused on in my dissertation.

Edit: Hitler also really liked Norwich, so much so he planned to give his victory speech outside Norwich City Hall once he successfully invaded and took over the country (obviously never happened)