Design RFID tag that works with Bluetooth
It's all about power.

Passive RFID tags work by transmitting power from the transmitter to the tag (typically three orders of magnitude more than the data signal), and then communicating with the tag at various frequencies (13.56 MHz, 433 MHz, and 900 MHz band being the most common).  The transceivers on these chips operate in the hundreds of µW range (probably 1 mW at the most).

As you mentioned, there are Bluetooth RFID tags (as well as WiFi), but they require active tags (a separate power source).  You cannot use passive tags with Bluetooth (even if they were redesigned to work at 2.4 GHz), primarily because of the power requirements Bluetooth requires to transmit, in the hundreds of mW (Bluegiga's WT11 module uses an average of 72 mA while transmitting at its lowest power setting).

Near-field RFID tags, like those an employee might swipe at a door, usually use an inductive loop for power, and operate at the lower RFID frequency (13.56 MHz), since the RF antenna and inductive loop can be combined into one antenna (in fact I've designed one myself).  These obviously operate at higher power.

Assuming you could power a Bluetooth transceiver using an inductive loop over a couple of inches, what's the point?  A passive near-field RFID tag can cost as low as 5¢, while a Bluetooth module may cost several dollars.  It is a complete waste to use a Bluetooth module, which is designed to operate over range of 100m, at a near-field range of an inch or so.