How come so many productivity apps for iOS make little use of drag & drop?

I was asking myself anyway when I got an app for pdf editing which made good use of drag & drop for file handling and worked much more seamlessly than several others I had tried out before. Especially the standard UIs for saving and stuff have a tendency to suck hard. Is there an obvious reason for neglecting drag & drop that I miss? Impaired usability on iPhone perhaps? On iPad it often offers an effective as well as intuitive approach.