Maybe because it's not free: 5$ a year for 4 devices but only the same type, you cannot install it on your Mac and your iPhone With only one licence.
So if you want to transfer between Mac and iPhone, it's at least 10$ a year.
And the other big problem - that is $5 a year just for your own devices. I have plenty of ways to move files from one machine to another, but if I have a file that I want to give to a family member sitting next to me, or a co-worker across the table the chances of him having the same service is zero unless I sell them on it. With AirDrop I just look at what hardware they have and immediately know if it is compatible.
