Is there a specific reason the Minotaur was kept in a labyrinth instead of a normal jail?
If not mentioned in the original writings, are there any speculations as to why go through the trouble of making a labyrinth?
The Minotaur was certainly the son of the Queen of Knossos, and possibly the grandson of Poseidon, the patron of Crete. You cannot just sling such a being in a cell; building a separate wing of the palace for him (specially designed for him by the leading architect Daedalus and visited at regular intervals by the best and brightest young people of Greece) sounds much better.
Royalty, by definition, is not treated like the rest of the population when it comes to crime or disease; there was a twentieth-century story of the 'Monster of Glamis', allegedly a member of the British Royal Family born with terrible mental and physical defects who was confined to Glamis Castle for life. The story had almost nothing behind it: what credibility it had came from the undoubted fact that if such a child had been born, it would certainly not have been been put in a normal asylum or seen in public.
I can't find any version of the myth that directly addresses this question, but the description of the labyrinth in Ovid's Metamorphoses leads me to suggest at least two educated guesses.
1) It's meant to prevent anyone from ever seeing the Minotaur, because Minos is that ashamed of it.
Mean-while the monster of a human-beast,
His family's reproach, and stain, increas'd.
His double kind the rumour swiftly spread,
And evidenc'd the mother's beastly deed.
When Minos, willing to conceal the shame
That sprung from the reports of tatling Fame,
Resolves a dark inclosure to provide,
And, far from sight, the two-form'd creature hide.
A regular prison would presumably still involve guards checking that the Minotaur is in his cell, while the labyrinth doesn't seem to require any guards.
2) The Labyrinth is designed to be so baffling and misleading that anyone in it would never find the way out.
Great Daedalus of Athens was the man
That made the draught, and form'd the wondrous plan;
Where rooms within themselves encircled lye,
With various windings, to deceive the eye.
As soft Maeander's wanton current plays,
When thro' the Phrygian fields it loosely strays;
Backward and forward rouls the dimpl'd tide,
Seeming, at once, two different ways to glide:
While circling streams their former banks survey,
And waters past succeeding waters see:
Now floating to the sea with downward course,
Now pointing upward to its ancient source,
Such was the work, so intricate the place,
That scarce the workman all its turns cou'd trace;
And Daedalus was puzzled how to find
The secret ways of what himself design'd.
Presumably the Minotaur can break through the walls of a regular prison cell, and possibly the labyrinth too (it's never described as having super-strong walls as far as I can tell). Ovid's description implies that even Daedalus would have trouble finding the way out, and Theseus obviously needed magic string to to do it, so it's likely that even if the Minotaur could tear down several walls that wouldn't be enough to stop him from getting lost.