Cauz da mojang crew haz sweg

ahhhh...

They made the new launcher, to make a database large enough(as HarrisonSJB said). The launcher was also recreated to hold the current, past, and future updates to the Minecraft(c) game. Also, Mojang made it manually downloadable, because that can be used as a statistic to see if the new update was largely accepted.

It's probably because Mojang has added so many things that the original launcher wasn't fast/big enough for 1.6. Because they're adding so many things, they made a new launcher.
I'm not sure if this is the actual reason, I'm just guessing, but that's as far as I can tell you.