Most UK seaside towns lost out to cheap flights to Spain. There's very little reason anyone visit Blackpool over Málaga. For the price of a depressing hotel in Blackpool you can get a modern resort in Spain with warm seas and reliably sunny hot weather every day.
There are a few seaside towns that managed to hold on, such as Brighton. They're mostly in the South where the climate and sea temperature is a touch more bearable and you can feasibly go there on a day trip from London.
Arguably, what's happened to seaside towns in the UK is really just a more aggressive version of what's happened to all smaller towns in the UK. The UK economy has become more centralised, particularly in London but also in other major cities. Towns have little to offer the modern British economy, as the engines that kept their economies ticking over moved to China and India.
Basically, they are victims of globalisation, both in terms of easy travel and also free trade.
[Un]fortunately, the UK government keeps them just afloat by pumping tax money into them via welfare, pensions and healthcare spending. Which arguably prevents them from completely failing, but possibly reinventing themselves to be more useful and relevant to the modern economy.