Ya'll want the "Simple answer?"
Cause their Squad/Plt/ Company leaders are cruel sadistic SOB's and wanna see the lil guy have to hump the heavy stuff. It's esp funny in Basic, or AIT and they give it to a lil 90lb female!
Honestly it is normally based on who is avail, and who has the mental and skill aptitude for use of the Crew served weapon, be it a M249, M240, M19, or M2.
But once in a while you do see the sadistic NCO issueing it out just to make someone sweat it out on a march.