Because of the many people that are going on Live and playing. Microsoft had not anticipated this many people subscribing to X-box Live or receiving a 360. But I'm sure their wallets were happy about it. I know it will take a long time, but you should leave your 360 on all night (with a cooler fan & not too many power cords plugged in) and wait for it to be completed. If you cannot do so, wait until the holiday season is officially over. I know it might take a few months, but this way it won't damage your 360. Or you can stop the download and try it again tomorrow.