The “MIDI HD” spec has been in development for a long time and manufacturers have been meeting to talk about it every year at NAMM. There have even been prototype devices created. I believe last year they said they’re pretty close to getting something finalized. So Ableton etc. are likely just joining to give feedback. If I recall correctly the focus is on having higher resolution parameters, higher transfer rates, and better synchronization.
I don’t think it has much to do with lack of availability of MPE in Ableton Live. Based on people I’ve talked to that’s more to do with the fact that the Ableton MIDI implementation is quite old and very tightly coupled into the whole program so it’s a lot of work to improve it.
Funny enough, as I was writing this message I got an email from the MMA:
Planned Major Update to MIDI Technology Attracts New Members to the MIDI Manufacturers Association (MMA)
The MIDI Manufacturers Association (MMA) announced that Ableton, Aodyo, Audio Modeling, Art and Logic, Melodics, Mind Music Labs, Native Instruments, OnSong, RnD64, and TouchKeys have joined the MMA to collaborate with other hardware and software developers on extending the power of MIDI technology. The planned update to the MIDI specification will support new levels of musical expression and make electronic instruments easier to configure and use.
Standardised in 1983, MIDI 1.0 has adapted over the years to support evolving operating systems and data transmission protocols.
This new initiative updates MIDI with auto-configuration, new DAW/Web integrations, extended resolution, increased expressiveness, and tighter timing – all while maintaining a high priority on backward compatibility. This major update of MIDI will support the development of a new generation of interconnected devices, and preserve the relevance of existing MIDI 1.0 devices.
“Our goal is to assemble a diverse group of companies of different sizes and business models and help them come to consensus on how to make their products interoperable using MIDI technology,” explained Tom White, President of the MMA.
Any company can join the MMA to participate and keep abreast of developments of MIDI.