I can't comment specifically on your question as to why, but I can say that the M1 is NOT always carried. Specifically, the US 3rd Infantry Regiment has units that carry the M-14 and the M-1903. The guards at the Tomb of the Unknown Soldier carry the M-14, while the US Army Drill team uses the M1903 Springfield.

You can tell the difference between the M1 and the M14 by a couple of features. First and most obviously is the length of the gas cylinder in relation to the barrel. An M1 has a gas cylinder that is the same length as the barrel while an M-14 has a longer barrel in relation. Secondly and not as obviously is that the M1 uses clip loaded ammunition - the action is held open and the ammo is shoved down into an internal magazine on a steel clip. An M-14 uses an external magazine. When you see the Tomb soldiers with their rifles, you'll notice that they DON'T have a magazine in the rifle.

The M1 and M-14 are similar - they were made by the same guy, they're both semi-auto. The major differences are the magazine and some internal improvements. The M1 and the M1903 are similar in that they are both stripper clip loaded, but the M1903 is really easy to spot: it's a bolt action rifle.

You are definitely right that the M1 is used frequently - you can find photos of the Caisson Unit marching at Obama's inauguration with the M1 (and I'm sure you could find them for other Presidents as well). As to why, I couldn't give more than a superficial answer. Good luck!