Heya Eso, hows it going?...I have a M-1 Garand as well as a fair bit of literature but I dont know if any of it answers your question.
But I'll throw in my guesses...Much cheaper (and simpler) than a box mag and automatically ejected, Much (much) faster than a reload with
strippers
or singles stuffed into an internal mag.
However the system limitations were addressed with the later M-1A which carried a box mag.
Lastly the system is pretty nifty, you can really crank off the rounds with little practice...and it was MILES ahead of the competition at the time.


