As the above post explains in such great detail, Dr. Faustus did not all other fields of academic knowledge. On the contrary, he was a great scholar and has actually exhausted all the knowledge that the methods of research in the 1580s (the time in which the play is set) could reveal to Faustus. He chose to explore magic because it was all that was left to conquer and because it led to knowledge outside the bounds of academic research. He chose magic because he thirsted for the immortality of having attained exhaustive knowledge.
Dr Faustus neglected all other kinds of knowledge because he considered them unfit for him. Initially he was driven by money but he realizes that having a complete control over one's mind gives you complete control over every element as the evil angel rightly points out
"Be thou on earth as Jove in the sky Lord and commander of these elements"
He overestimated the power he could get through the supernatural, but in the play Marlowe gives us a subtle hint where at the court of the king Faustus accepts his limitation in not being able to bring the true substantial bodies of Alexander the great and his paramour.But in the first scene he grumbles over medicine
"Couldst thou make men to live eternally Or being dead raise them to life again?"
Magic could only bring the ghosts but Alexander is not in a position to live his normal life after death. Thus necromancy is no better than medicine and he is clearly duped.
The dramatisation of the Faust story by Marlowe in Dr.Faustus is to show man's inordinate craving for power & knowledge. Faustus, an exemplary scholar, strikes a deal with Devil for twenty-four years of all voluptuousness and supreme power, on termination of which his soul would be taken away to hell. Faustus embraces the study of necromancy to gather power through the conquest of the whole world of knowledge.
All the major plays of Marlowe--Tamburlaine, The Jew of Malta, Dr.Faustus--deal with exceptional personages hankering for world-conquest. Tamburlaine slaughtering his way to military conquest of the world; Barabas, the jew, aiming for a world-conquest through amassing fabulous riches; Faustus runs after the same conquest through the mastery of knowledge. All the Marlovian protagonists suggest the irony of human greed, greed activated by hubris, grred to possess enormous power, political or monetary or intellectual.
Black magic claims to perform anything, the most fantastic & the most impossible. Faustus chooses magic driven by the delusion that he can achieve anything. He barters away his soul('the eternal jewel') to Devil(the common enemy of man'). Marlowe's play deals with the problematic of sin/evil. Satanism, Sinfulness, Salvation are the main motifs involved in Faustus's choice and the ironically destructive outcome of the choice. Black magic is Devil's knowledge, and Faustus's choice of it allegorises the Fall of Man, and even prior to that, Satan's own fall from Heaven.
If I were Faustus, perhaps I would have made the same choice, for that is the basic irony of being Faustus.