Primarily, the Romans had access to the major commercial outputs (gold, ivory, and to a lesser extent slaves) of the Middle Niger and Western Sahara in interior Western Africa via trans-Saharan trade routes, though the total capacity of trading caravans for most of the Roman period was limited because camels had not yet been introduced into Western Africa. The early states of the forest belt and Senegambia were not yet strongly connected to the Middle Niger and trans-Saharan trade, but they would have offered relatively little in addition to what middle Niger states and population centers could provision.

The Portuguese were motivated to sail down the Western African coast for a variety of reasons, none of which really applied to the Roman Empire at its height. Rome controlled both sides of the Mediterranean and thus had access to goods that came into Northern African trading communities like Sijilmasa and Tripoli. Whereas North Africa was controlled by Muslim rulers in the 15th Century after Portugal came under the control of Christian rulers. The Portuguese were interested in reaching the Indian Ocean world via travelling around Africa because again, the Red Sea access was controlled by Muslim rulers. Rome had other points of access to overland trade from South and East Asia. And Portugal's geographic position gave it a strong maritime orientation towards the Atlantic, whereas the Romans focused primarily on the Mediterranean and on hugging the coast northwards to Gaul and Britain.

Which is another factor: sailing southwards to Western Africa was a genuine technical challenge for Portuguese sailors and would have been more so for the ships of the Roman period. It took quite a bit of trial and error to figure out how to sail southwards along the Western African coast--it involved a lot tacking out from the coast and then coming back again. Roman naval history is out of my expertise, but I believe that the Roman Empire in fact maintained almost no seafaring navy after the early imperial period, using ships on rivers and near coasts for transport and local military action. Perhaps a classical specialist can say more on this point. So not only did the Romans (and various merchants active within the empire) have no incentive to connect to Atlantic Western Africa, they didn't really have the capacity.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!