Because it's dangerous ransomware and you must have clicked something or used the keyboard - just one stroke or action activates it.
If you had powered off immediately you might have escaped it.
Moved to Malware Discussions > Home User Assistance.
Have you tried initiating System Restore in Safe Mode to go back to before all this started?
If you've read the other threads then you'll know that no A/V can stop these things. It's the way they work, involving user interaction, no matter how trivial such as a mouse click or a key stroke, to activate.
There is a definitive guide here: http://www.bleepingcomputer.com/virus-removal/cryptolocker-ransomware-informatio n
Note the blue box lower down with a link to the thread on that infection where you may be able to get some help.
McAfee are aware of these things and in fact have a tool that catches some but the trouble is that variants appear constantly.
See Stinger in the last link in my signature below. It wont decrypt files however, that's where BleepingComputer forums may be of help.
Unfortunatel regular A/V's rarely are effective against these things.