Why did Julius Caesar name Octavian as his heir and successor?

Caesar named Octavian as his "private" heir, not his "political" heir. ... When Octavian become Augustus and the first roman emperor he did assume a position and powers that Caesar could never have imagined, much less specified in his will. Augustus was born Gaius Octavius on 23 September 63 BC in Rome. In 43 BC his great-uncle, Julius Caesar, was assassinated and in his will, Octavius, known as Octavian, was named as his heir. ... His powers were hidden behind constitutional forms, and he took the name Augustus meaning 'lofty' or 'serene'.

Making Caesarion his successor would have made Cleopatra queen-regent of the republic and would have established the kind of autocrat that Caesar would not have been very fond of. Caesarian and Octavian were very different people, even as children. Raised by his mother Cleopatra and propped up as a god-king in his homeland, Caesarion was quite likely to become what Caesar did not want. While his mother was a romantic for the old Egyptian ways, Caesarion might have become like his uncle Ptolemy XII. A man who believed it was his divine right to rule over his people and didn’t give much thought to them.