The reason for the new nickel is all about money. Ha ha.
But seriously, the treasury rakes in money on everything they issue. Coinage is no longer actually worth anything; hasn't been since they took the silver out. Nickels have never been worth a nickel; they just represent a nickel, as paper money represents an amount. So every time they produce a nickel, they're making money...er, I mean a profit.
Now, changing the coin does one thing: it attracts collectors. Collectors remove the coins from circulation, and more have to be made. The ones taken out of circulation are even more profitable than the others. It's like stamps sold to collectors; they get the 37 cents and don't even have to deliver a letter. Add to this the profit on special "collector products" like proof coins, special mint coins, etc. Check out this website and see how much a new roll of quarters will cost you from the mint: U.S. Mint Website
Besides, coins used to change a lot more often than they do now. I remember collecting coins as a kid; you could find Roosevelt, Mercury, and even an occasional Barber dime in your change. Buffalo nickels were still around. Three different fifty cent pieces. Hell, it's about time they mixed it up some...