That's a great question. We wish that it were possible to use inexpensive Bluetooth adapters for iOS devices, but unfortunately that's not possible in many cases.
The reason for this is because Apple requires a certification for all Bluetooth devices before they'll allow them to work. It's called "Made For iOS" or "MFi" for short. This is both a good thing (in that it ensures that almost all allowed devices will work well with your Apple device) and a bad thing (in that it a lot of manufacturers don't want do the certification, so fewer devices are allowed.)
Most low-cost generic Bluetooth OBD adapters unfortunately don't have the MFi certification, and won't work with iPhones - and, conversely, most Bluetooth OBD adapters that do have the MFi certification seem to have their own, proprietary communication protocol.
This is why there are currently limited options. However, we are always looking for more Bluetooth OBD adapters for iPhone -- and they'll be added to our devices page as soon as we can confirm that they're up and running.