There's ultimately a lot of risk and reward in sending them to Wakanda. It's a place that's very believable as far as hiding the gem, and it's a place that can hold Thanos forces off (it's the only place that flat out beat Thanos forces at least in round one). Combine that with the obvious advantages of pitting 2 of Atlantis enemies against each other and it's easy to see how overall it's attractive choice.
There are downsides as well. And that's the gamble Namor is making here. He's likely assuming that earths forces will eventually rally and drive Thanos forces away. Ideally from Namors perspective, not until after Thanos can do some damage to Wakanda... but ultimately I'm sure he doesn't want Thanos around anymore than anyone else.