It was my understanding that Ash is not a good wood for chopping boards like this as it's quite hard and porous (so catches food that can go mouldy). Why was Ash used?
Yes, ash is somewhat more porous than hardwoods such as beech or maple, but it is a sensible choice for chopping blocks as the wood is particularly tough, impact-resistant and durable. With solid, oiled boards and regular care (cleaning, oiling), the more open pore structure is not a problem in practice.