Why did Dowlais ironworks Merthyr Tydfil shut down?

The ironworks didn't shut down as such, it had been converted into a modern steelworks, bought out the nearby Cyfarthfa Works and entered into partnerships to create the world famous brand of Guest, Keen & Nettlefolds, but new foreign competition, especially the huge German and American combines made continued investment too expensive given the cost of necessary imports, particularly from their Basque iron ore mines, so it was dismantled and moved to the coast at Cardiff East Moors on the Severn Sea (Bristol Channel), 'The Dowlais Works, Dowlais' became 'The Dowlais Cardiff Works'. Back in Dowlais itself the main body closed down in 1936, but the Ivor Works foundry, named after Ivor Bertie Guest, Lord Wimbourne, the son of Sir John and Lady Charlotte, was continually modernised and carried on making ingot moulds for export until it finally closed down in 1987.