Why did Doctor Faustus neglect all other fields and choose magic?
Faustus views the traditional disciplines of "analytics," "economy" and "physic" to be too confining for him. They do not allow him to satisfy his true ambitions. Magic, on the other hand, offers wealth, power, and esteem that are unattainable for ordinary scholars. Contemplating the benefits that would come to a scholar who had mastered "the metaphysics of magicians," Faustus fantasizes that magic would allow him, unlike ordinary men, to "raise the wind" or "rend the clouds":
O, what a world of profit and delight,
Of power, of honour, of omnipotence,
Is promis'd to the studious artizan!
These speeches by Faustus attract the attention of Mephistopholis, a minion of Lucifer, who reaches an agreement with the greedy doctor: Faustus will give his soul to Lucifer, the embodiment of evil, in return for earthly power and riches. It is ambition and greed, as well as an inflated sense of self that drives Faustus to make this terrible bargain. He neglects the traditional disciplines of medicine, economics, and theology because he thinks they are not big enough for a man of his intellect and ambition. This sense of hubris flies in the face of God and would have reminded Marlowe's audiences of the necessity of humility.
The distinction between magic and what we would today call natural science was somewhat blurred at the time when Marlowe wrote Doctor Faustus. A lot of what would now be regarded as pseudoscience—including astrology, alchemy, and divination—were perfectly respectable academic disciplines in Renaissance society. Doctor Faustus's embrace of magic should be seen in the context of this background.
The difference with Doctor Faustus is that, unlike other scholars, he accepts no authorities in his search for fame and glory. In his opening speech in act 1, Faustus reels off a list of academic disciplines whose continued study he now rejects because he must operate within the limits set by the great figures of the past. Faustus is possessed by a spirit of hubris; he will not allow anything to stand in the way of his overweening vanity and pride. He will achieve fame and riches come what may and if this means rejecting God and the accumulated wisdom of humankind, then so be it. The problem, then, is not that Faustus has embraced magic; it is that he has embraced it in a spirit of arrogant, impious pride and will shamelessly use it for his own selfish, wicked ends.
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.
As previously stated, Dr. Faustus is bored because he's exhausted all educational avenues which is shown in Scene 1 with Faustus's opening soliloquy. The soliloquy ends with Faustus saying, "A sound magician is a mighty god./Here Faustus, try thy brains to gain a deity." (Lines 62-63) Faustus realizes that the only thing left for him to do is take on the power of a god and the only way he sees to do this is by mastering magic.
Marlowe used the Good Angel and Evil Angel as a type of stream of consciousness which was a seldom used literary method at the time to show the way in which Faustus struggled with the desire to become a magician. Ultimately, Faustus follows the advice of the Evil Angel and through a devil named Mephastophilis, (sometimes spelt Mephistopheles depending on the publisher) gains the use of magic for 24 years in exchange for his soul. Though he has many opportunities to repent, he always either ignores his chance or is too busy taking pity on his own fate.
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.