Because lamb is in ranched New Zealand.Lamb producers in the UK can't compete with their low cost production even taking into account the cost of getting it here and also the fact that it's available all year round whereas here we have spring and autumn lamb.Also sheep in this country are one of the only animals that are not reared intensively .
People want cheap food and that is what the supermarkets provide They buy in bulk and then pay our farmers a pittance and whack up the profit margin and not only for meat.
I would rather pay a bit more and have something tasty now and again than buy a lump of frozen meat I don't really know the provenance of.
There are cheaper cuts of lamb ...neck for example which makes lovely hotpot.
Support your local farmers markets and butcher ..it's often cheaper and much better quality !!