Because it has a more explicit interface (vs. Dropbox's "implicit" interface.)

That is, for what I can see with Receivd the user explicitly "opens" an app (as they're used to doing for almost anything else on their computer), which in turn clearly states "drag and drop files here to share."

With Dropbox the user needs to understand the concept of a shared folder that "magically" syncs with other computers. For many users this is an alien concept and may confuse them. I know of many people who have a Dropbox account, yet use the service exclusively through the web interface, simply because typing a URL, logging in, browsing for a file, clicking and downloading it is a concept they "get."

There's no such thing as "one size fits all" in this world---specially when it comes to user interface design.

Edit: I should clarify I do not work for Receivd, nor have any connection with the team, so I don't speak for them. This is just my assessment as a UI designer.

Minusfive's point of view tracks with years of working with non-technical customers, many of whom have a mental model suggesting that files "live in" the application as opposed to being in a folder somewhere.

I don't know how far dragging and dropping files is going to get them (many of my users could not locate a file if it bit them on the nose), but hey, it might work.