Very simple! Does it not taste a bit bland without any spices? What are the traditional spices used in Arab cooking?
They have all sorts of spices - varying from region to region. But most would not add too many spices to barbecued/roast/grilled meat. If you have good quality fresh lamb it really doesn't need to have it's taste smothered by too many spices.
Fresh corriander and salt are all that is needed!
Anyone been to a good Lebanese restaurant and eaten the variety of barbecued/roast lamb dishes? If so you will know that good meat only needs cooking well and it is delicious on it's own.
Mutton on the other hand needs the life stewing out of it to make it tender and then loads of spices to to make it tasty.
Fresh lamb doesn't need that.