Did they know that at the time? Would the usual train of thought be that the blacks can handle the tropics since that is where they came from originally?
At first they thought that the diseases etc wouldn't be a problem, but they found out pretty quickly that it was. Fifty percent mortality will do that to you.
Moving Liberia wasn't thought to be enough of an issue because, mostly, not all that many people went there anyway. I think the total number of freed slaves etc who went back to Liberia was only about 10-15,000. This doesn't include the people freed from illegal slave traders (who were often settled in Liberia), who had at least some more resistance than the Americo-Liberians.
The reluctance to move was also because Liberia was a private enterprise, not a government project. If large numbers of people are moving to Liberia (100,000, say), that implies more government support, at least, and I doubt that they'd be popular if they supported it when half of the migrants were dying...
The problem, as mentioned, with Mozambique (well it wasn't called that at the time) would be the distance.
This distance is bad, but having most of your migrants die is worse. Natal is marginally better, assuming the Boer Trek doesn't happen, or somewhere in southwest Africa.