Because, as is often with these pieces of art, they hold hidden messages. There's a reason people pay $100 million for a piece of art, that mechanically could have been painted by a talented college student.

There was a whole "counter culture" movement, that was in protest against the catholic church. They held "ancient" religions, such as those of the Babylonians, Jews, Egyptians, Greeks, as more valuable than those of the catholic church. But, the Catholic Church(or Monarchical Churches, or the Eastern Orthodox Church) were in Power for hundreds, and sometimes even thousands of years, depending on the location.

So, artists in the renaisannce, and much before, had to hide their political messages, which ran counter to Catholicism, in their art. Jesus is making a very specific symbol with his hands. And the Crystal ball is to symbolize jesus's initiation into the "Mysteries"(which were essentially the name of this anti-catholic, pagan religion).

When you see an egyptian Monolith in NYC, or a Pyramid on the back of the dollar bill, or various Roman/Greek/Egyptian Symbols in DC, it's no coincidence. Many of our founding fathers(and the French who supported us, by giving us the statue of liberty, for example) were free masons. George Washington was a mason.

So, these paintings have become symbolical, for the anti-Catholic religion(which could be called paganism... but I don't know the extract term, as it's fairly hush hush). These paintings would be like buying a piece of clothing that Jesus wore, to Christians. Or to buy some of the oil that was used in Hanukkah. That's the reason these paintings go for STUPID amounts of money. Plus, the people who utilize these pagan beliefs, tend to be very rich... so the demand is all from rich people, which drives the prices up further.