It is a hardware limitation because the existing iOS implementation uses BT4.0, therefore the Mac must as well for compatibility.
BT4.0 was a good choice for iOS. It helped to keep battery consumption low while allowing passive detection of incoming AirDrop requests, rather than having to poll regularly (wasting battery) or depending on the user going into an active receive mode (as is done with legacy Mac AirDrop).
It is disappointing that USB BT4.0 adapters are not supported for AirDrop on the Mac. It is most likely to be a deliberate choice by Apple to only support their hardware, at least for the initial release. It would be nice if they relax this rule in later updates, or if it is possible for a third party kernel extension to enable AirDrop with a third party USB BT4.0 adapter.
There are existing OS X features along these lines, e.g. with a simple kernel extension it is possible for Thunderbolt docks to enable "extra operating current" negotiation for their USB ports. The CalDigit Thunderbolt Station uses this to supply more current to an iPhone, iPad, iPod, Apple keyboard or USB SuperDrive. The kernel extension requires OS X 10.9.2 or later, so Apple added support for this after the initial release of Mavericks.
Submitting feature requests to Apple and/or USB BT4.0 adapter manufacturers might encourage Apple to allow a similar mechanism for AirDrop support.