A little bit of everything.
XBox Live - Still the best service imho, tho obviously it's costing me $30 or less a year (Thanks to recent deals through Amazon and Sears). Every game has some sort of online function these days.
Slight Brand Loyalty - The Original XBox system was a beast, both in size and power. It was a durable system that offered the best in graphics (most of the time) when games were multi platform. I was really happy with the original XBox so I figured the 360 would be just the same.
The time it was released - The 360 was out the door first before the Wii and PS3. The temptation was just way too hard to pass up. I wanted to jump in.
I got my 360 two months after launch due to supply issues. The 720/whatever it will be called will most likely be my only next gen system for some time due to time constraints and money.
It's just that simple.
Essay Pro