Because it's all I need and in hand.
Aside from that....
The magazines are smaller and easier to carry.
It doesn't weigh much more than a gnat's whisker.
It kills pigs DEAD in my hands, people aren't any tougher.
It's the first CF rifle I ever fired.
I done wore out one -16 in combat, looking for a new adventure.
I can deliver a better butt stroke with the M1 than a M4.
Need more???? Well, OK, since you asked....