I never found out the reason. Maybe they figured that was more pratical using the"en bloc" clip since the clip is ejected when empty so the rifle is ready to receive a full clip and clips take less space(in my opinion) than box magazine and are also very cheap to manufacture. Still the drawback with the Mannlicher type clip is that without clip, the rifle is a single shot one(same goes for Carcano and berthier rifle).Another great thread Joce.
Have you ever found out why the US Army went with 8 round enbloc (Mannlicher-style) clips rather than the 20 rnd magazines used in the BAR?? After all, the BAR in US service used the same ammo, and using 20 rnd magazines would have meant that there was no need to manufacture and issue mag pouches and enbloc clips specifically for the Garand.
Even Hatcher's Notebook doesn't say why enbloc clips were used and not detachable magazines.