[yholio]

Because Dropbox is a file storage service, not a file distribution system. The typical use case is that an uploaded large file will be downloaded a single time by someone else, at different times when computers are online.

This requires an intermediary system with very high availability and performance, which in P2P can only be implemented by duplicating data on a dozen or so systems.

Since files must be private and there is no commonality like in BitTorrent, it follows that running a P2P Dropbox would consume 5-10 times the storage and bandwidth of it's centralized counterpart for the same useful service.