I would assume it has more to do with how many intermediate places within the continent you want to connect to. If you're connecting two cities on the African coast, and there's really no infrastructure in-between that needs (will pay for) connection, and the roads in between are poorly maintained, it's probably more economical to go by sea. Someplace like the US, if you're running cable from say Boston to NY, first of all, you also want to wire up all the towns and cities in between, and you've got interstate highways so your service trucks can easily get to all parts of the cable, you've got existing utility tunnels you can use, etc. In that case, going by sea is an unnecessary expense.

If you take a look at an Internet penetration map[1] you will see that most of the cables around Africa have only a few big "customers": Morocco, Egypt, Nigeria and South Africa. If you compare that map with a GDP map you will also see why: poor countries, without terrestrial infrastructure and some with unstable political situations.

[1] http://en.wikipedia.org/wiki/File:Internet_Penetration.png

I noticed quite a cluster on the South American coastline - Brazil-Argentina coastline looks like it's got 4 or 5 that go nowhere but that coastline. You'd think Brazil was stable and prosperous enough that they'd go with landlines.

The population is strongly clustered on some big cities in the coast, the roads aren't super safe, and the terrain around the coastline is not flat, so it's probably just cheaper to lay them out in the ocean.

Of course telecom's is generally broken up by country, and most countries with a coastline have at least one undersea cable.

Before some of the recent cables East Africa mostly had to get by with satillite internet. That wasn't fun.