There might not be anything mechanically wrong with the vessels being scrapped, but with the building boom in the last few years charterers can get modern vessels for a cheap price so less advanced vessels are not really attractive.
As Ombugge mentiones on many modern vessels are a lot more fuel efficient so charters save money on fuel, newer PSVs have bigger decks and dry and wet bulk tanks so less ships are needed. Newer AHTs are more fuel efficient, have bigger storage winches for fibre ropes and wires, along with bigger chain lockers for storing chain. They have higher bollard pulls for pre-tensioning anchors to much higher tensions when pre-laying mooring systems, and rail-cranes with manipulators to make deck operations much faster.
Many older OSVs have nothing physically wrong with them, they are just at the end of their economical life as there are so many other vessels that can do the job a lot better for the same price.