>>64138944 (OP)
Cost, weight and logistics.
Theoretically they could have designed the M1 to use BAR magazines, since those did exist already but they would increase the overall weight carried by a rifleman considerably. Having the ammunition loaded on clips meant you have a small disposable bent piece of steel rather than a much larger/heavier box with that needs to be reused. If an M1 clip gets damaged, use another one. If a BAR magazine bends a feed lip, it can be chucked too but that's a bigger thing to replace.
Moreover, unlike magazines they could pre-package sealed ammunition cans already loaded on clips in bandoliers. Storing magazines pre loaded like that invites either springs or feed lips deforming (less of an issue nowadays with better magazine design, but a significant concern at the time). M1 clips can be stored loaded pretty much forever.
The limited capacity and reload wasn't considered a problem. 8 is less than 20, but it's a lot more than 5 which is what almost everyone else was using. The reload was also faster than changing a box magazine since the clip would be ejected automatically, requiring only the insertion of another one. Since the clips are disposable, there was no need to retain them either. This simplifies what the soldier has to keep track of during battle. As long as he has ammo, he has clips. As long as he has ammunition and at least one clip , he has a working rifle.