Upvote:5
The answer lies in two considerations:
First, the specific operation that led to the Battle of Magadishu was designed as a capture operation: the Americans should have landed, capture and go back. All of that fast, with special operators. In this context, artillery or fixed-wing air support was supposed to be dangerous in terms of collateral casualties, for minimal gain since their was no time for one of the special operator to direct the fire support/
Second, there was not that much available in Somalia, or in close-by bases (such as Diego Garcia: you'll find B-52 there, but not A-10 nor artillery)
So the answer to the "why" is: the Americans did not think they would need that, so they did not prepare such assets in Somalia. For the specific battle of Magadishu, they did not anticipate the need either.
Now why did they think that?