[ubs28]

Can't they also include Airplay 2 and UPnP/DLNA on top of bluetooth for even better sound quality?

 

[genck]

Because non audiophiles don't like cords (aka everyone - the "market")

 

[rkw]

A reason for "why so many" is the ease and low cost of implementing bluetooth. Headphone manufacturers can use off-the-shelf chipsets that combine bluetooth receiver, dac, and headphone amp. Some chipsets even have integrated noise cancellation circuit. Furthermore these chipsets are optimized for portable use, and able to run at low power to prolong battery life. This kind of infrastructure for portable operation is not available for Airplay and DLNA, and frankly there might not be enough market demand for a headphone manufacturer to justify developing their own circuits.

I would also question how much benefit would come from using Airplay or DLNA. Wireless headphones operate with severe constraints — battery, receiver, dac, and amp are all miniaturized, lightweight, and built into the earpiece. The digital input stream is probably not the bottleneck in sound quality and it would be more productive to concentrate on improvements elsewhere.