There are two reasons I fan think of as to why buying digital rather than phisical. First, it's more convenient in terms of place, toi dont have to have a big place.
And, if it's like here in France, it could be cheaper in digital. Here Digitals are half the price
I remember reading that digital sales had surpassed book sales in japan and maybe people are just switching to digital and we have no idea what this represent in term of sales since we haven't got any data on digital