It was purely a business decision on the part of both the gun and the munition manufacturers, and a good one if you think about it. Damascus barrel manufacturing was a very time-consuming, labour-intensive process (ie. expensive!), and by 1905-1910 the compressed fluid-steel process (perfected by Sir Joseph Whitworth) had evolved to a much more reliable mass-production enterprise (ie. significantly cheaper).
Nitrocellulose-based powders (nitro) had also been slowly taking a larger market share of the propellant market away from black-powder boys during roughly that same time-frame, due in large part to nitro's far more stable (ie. safer) manufacturing and handling properties(blown-up buildings are bad for business), not to mention its vastly different other qualities (faster, cleaner, more dependable ignition over time, etc.) By marrying the two relatively new products (fluid steel and nitro powder) they arguably had a much better product to sell. They then made that market even-better by inferring that all the weapons made before that specific time were not only inferior, they were downright dangerous to use. Instant obsolescence!
To be fair, the world was a much-less sophisticated place then, and shotguns were more tools than a sporting device. Most, if not all such weapons in this country were used with little consideration for their design-limits and many did-indeed blow-up. The primary culprits of this were poorly-made, dirt-cheap, mostly imported trade-guns(from Europe, primarily from Belgium) but by then, all Damascus-barrelled firearms had become suspect.