Shadowmourne shards have a reduced droprate to discourage players from going for the free route to get shadowmourne and instead purchase it via shop on icecrown. This is fine when the option is available.
For Lordaeron the option is not available and all the bosses have abyssmal droprates for shards, even on heroic, with only Lich King giving a +1 shard every week and often being the only shard to drop even in 12/12hc raids.
