Because, IMO, the two haven't been apart long enough to come back to a relationship with anything new and fresh, and the same problems they have faced before are bound to crop up. Granted, this is all contingent on the two of them rekindling their romance, but I think that them seeing each other in Africa could very well send them right back into the relationship they just ended. If Chelsy is the right girl for Harry, they will be together in the end.
For now, I think it would probably be best for both of them to give it a rest.
That's just the problem with royal relationships, though, isn't it...someone can be "the right girl" but not the right "royal bride."
My impression is still that Harry and Chelsy had a real connection and were in love, and at the same time, Chelsy probably has no interest in becoming a royal bride or even living in England. So their relationship really can't go anywhere, but until one of them finds someone else, they will probably keep going back and forth to each other like this...whether as friends or romantic partners. To me it's sad when that happens (that a royal falls in love with someone who doesn't fit in with the royal family).