There is no one why. It's lots of little reasons for and relatively few against. If you benefit enough from the fors and don't suffer to much from the against then you'll probably enjoy the switch.
Good quality well designed hardware
Stable Secure OS without lots of techy knowledge and/or maintenance
Fewer OS specific serious malware threats
Great bundled iLife suite to help make Mac your digital media hub
Great range of productivity software to help you get stuff done quickly and enjoyably
Strong emphasis on simplified user interaction (although this can be a stumbling block for experienced windows users)
Some programs you may be used to not available
You may use windows at work so will be always stitching back and forth
If you shoot JPEG, iPhoto may offer everything you need for organising, displaying and simple enhancement of your piccies. It comes with the iLife bundle. Photoshop CS/Elements avaliable for pixel tweeking.