Two main reasons, and both are common sense:
1) Inflation, as many people have said
2) Increases in demand for Broadway shows while supply has been stagnant
On the second point, yes, I know that there are great shows off-Broadway and off-off-Broadway, but when you fly in from out of state or even out of country to see a show, you're seeing a BROADWAY show. The tourist demand has increased by virtue of population increasing, if nothing else, over the years. But you can't just build a new theatre in the theatre district. There's no more space. Supply is mostly stagnant, so prices go up.
There's also a more complicated third reason in that they don't offer the same prices to all consumers. They offer the best tickets for super high prices because people will pay them and then sell balcony seats to those who can't afford orchestra. They manage to make every consumer pay close to their reservation price (the highest they'd be willing to pay) by offering seats at varying prices and qualities. Also through a plethora of discount programs.
BROADWAY: The Cripple of Inishmaan, This is Our Youth, If/Then, Hedwig and the Angry Inch (x3), Cabaret (x2), The Real Thing, A Gentleman's Guide to Love and Murder, Chicago, Les Miserables (x2) Disgraced, Finding Neverland, On the Twentieth Century, Wolf Hall Part I, On the Town, Fun Home; TOURING: Jekyll and Hyde, The Book of Mormon; LOCAL: The Twilight Zone, Anne Boleyn, Death and the Maiden, The Lying Kind, Chorus Line, Stupid F**king Bird