Because of the drop rate
You will have to start with Hellfire Peninsula when moving on to the Outland Mining. Hellfire is a small haven for miners and herbalist as well. The ore and herbs found there are not as valuable as Adamantite, but collecting Motes of Fire and smelt the ore is a great way to make money with LOTRO Gold. You can prospect Fel iron Ore, but it is not recommendable, because of the drop rate for rare gems is infinitely lower than Adamantite.