A Bluetooth device is much more complex than the proprietary wireless devices. The complexity is largely because Bluetooth hardware must support many different types of devices all with differing requirements. A proprietary keyboard dongle doesn't have to be able to deal with the audio traffic coming from a microphone or going to a headset. Furthermore, it doesn't have to support proper identification and authorization of each individual device; many of them will accept input from any compatible keyboard that happens to be near by.