Because this resource isn't as old as the current question is, I am sure it is the answer.
The limitation for sending output to multiple connected bluetooth devices is not about a feature. Its about how Bluetooth enabled communicate with each other.
Bluetooth, from day one, was designed for a one-on-one communication over short distances for short periods of times i.e sending an image, short video clip etc.
The use of Bluetooth has evolved and as such support for multicasting is much wanted.
Quoting from this XDA forum post:
The reason why bluetooth doesn't support Multicast is because it doesn't have the bandwith to support High Quality streaming simultaneously no would the chips really having the processing power but that could be fixed, but the main limitation is the lack of available bandwith specified in the 802.11 standard for bluetooth.
... it is clear there's a limitation, and hopefully work is done in that regard to allow support.