Bluntly put, it's Christopher Columbus' fault, and all those who followed him.
He was a Catholic (regardless of possibly Jewish connections historians recently attributed to him) and as such saw himself as a defender of the church. The name he chose indicates as much -- 'Christopher Columbus', basically meaning 'Defender of Christ' -- is only the christianized name he signed with, it is believed that it was not his real name.
Columbus The Prophet
All his writings indicate how he dedicated his accomplishments to the Catholic religion, and just before his death he even wrote a book of prophecies in which he described how he was the fulfiller of biblical prophecies.
However, he was catholic -- or 'Christian' -- only through a historical fluke. Spain, the country that sponsored his trip, was Christian only because they had just finished beating back a Muslim invasion, literally just months before Columbus set sail.
In fact, almost 800 years earlier a Moor invasion had conquered all of the Iberian peninsula and was pushing further north into Europe. While Christians were tolerated but had to pay a tax for being non-believers, the land was slowly being converted and for anyone to make any headway, turning to Islam was the way to go.
It is conceivable that eventually all of Iberia would have turned into a purely Islamic caliphate, had it not been for the 300 Asturians.
The 300 Asturians
Indeed, a lot of historians (particularly Asturian ones -- but who can blame them) insist that the actions by the 300 brave Asturians during the height of the invasion turned the tides. Rising up against the invaders (blame those darn taxes) and led by one Don Pelayo, they chalked up the first decisive win against the Moors. It was enough to inspire. Over the next 800 years or so the Christians gradually beat back the Moors, until the last remnants officially surrendered to Queen Isabella in 1492.
A few months later a Christian Columbus set sail.
Speculations abound as to what would have happened if the 300 Asturians had not risen up. While history is a fickle thing, fact is, had the Moors not been beaten back eventually, a Muslim Columbus (or another Muslim explorer) may have found the New World.
And America would now be a Islamic caliphate.