To tell the truth the most serious reason is because the task is difficult. The iPhones Bluetooth subsystem is a fully undocumented black box. There are number of high-level function available to Apple's application in unsupported frameworks, but there is no low-level functions available at all.
A2DP can be implemented on a system where the following requirements met:
- Raw system sound stream can captured by the application or a driver.
- A L2CAP level sockets interface is available for third party application.
- Device has enough computing power to encode sound stream using SBC encoder.
- Bluetooth Host Controller connected to the CPU using high speed interface. 115K serial line is not enough :)
- All Bluetooth subsystem is quite bug free and stable. A2DP generates a lot of packets and sensible to delays due to realtime nature of audio.
Regarding iPhone we have the following status:
- I have successfully captured sound stream in firmware 1.1.4. This was broken in 2.0
- I was able to send and receive HCI packets. L2CAP interface so close in 1.1.4. This was broken again in 2.0
- CPU power is more than enough in iPhone.
- HCI connection isn't perfect, but preliminary test shows this should be a bottleneck.
- Have no idea about this yet :)
Will we continue development? I hope so.