For the same reason American men take their young mistress playtoys to Cancun or Bermuda or Aruba and not to Cleveland.
Their young very PREGNANT "mistress playtoys"? And they purposely have their children there, in some country that's neither theirs nor the mother's?
One reason is that his father was Muslim and was probably Arab as well as African. Kenya is mostly Christian, but the Coast Province, where Mombasa is located, is decidely Muslim. Being a coastal trading center it probably had better facilities than Nairobi where Obama sr. would have had no ties.