Why wouldn't it be? You probably imagine North Africa as just sand everywhere, but that's emphatically not the case. Hot climate isn't all that bad, and it's not as dry as you'd think.

It had to be decent agricultural terrain to support one of the superpowers of the time: Carthage. Carthage developed strong agriculture in North Africa and the region was better watered than it is now. The deforestation of the northern Mediterranean coast in Roman times changed the water flow to the detriment of the area.

Don't be fooled, though, the Maghreb is still quite fertile and just because most people think of North Africa as mostly desert doesn't mean all of it is sand with no water. A large portion of the area is still very good agricultural land. Due to depletion of some of the natural resources and climate change influenced by Roman actions, it's not as fertile as it used to be in Roman times, the areas settled at the time - and now! - were usually close to the coast and that area is, in fact, great for farming even now. The fact that you could essentially farm along the coast also made it easier to transport grain. And once you add the Nile, it becomes pretty clear.

In fact, people have wondered that in the past, too - /u/bitparity and /u/at_dawn_they_come have chipped in. They provide additional points.