As a disclaimer: Yes, I know Thunderbolt (2) is multi-stream capable of up to 6 devices in a single chain. Yes, I know you can add a single DP monitor at the end of a Thunderbolt daisy-chain as an extended desktop. That said...
What prevents Thunderbolt from supporting DP 1.2a (mst)? I understand that the primary difference is Thunderbolt provides power along with its data connection. However, looking at articles like Wikipedia it makes it sound as though it shouldn't have any issue allowing a DisplayPort-only daisy-chain.
For example, currently you could create such chains via Thunderbolt:
Mac --TB(2)--> TB Display --TB(2)--> External RAID --mDP 1.2a--> DP monitor Mac --TB(2) #1--> TB Display --mDP 1.2a--> DP Monitor \----TB(2) #2--> TB Display --mDP 1.2a--> DP Monitor Mac --mDP 1.2a--> DP Monitor
But you can't do this:
Mac --mDP 1.2a mst--> DisplayPort Monitor --mDP 1.2a mst--> DisplayPort Monitor
through a Thunderbolt port and configure them both as extended desktops (the second monitor will mirror the first one).