If Thunderbolt is a successor to DisplayPort, why can't it support DisplayPort multi-streaming? 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)--&gt; TB Display --TB(2)--&gt; External RAID --mDP 1.2a--&gt; DP monitor

Mac --TB(2) #1--&gt; TB Display --mDP 1.2a--&gt; DP Monitor
 \----TB(2) #2--&gt; TB Display --mDP 1.2a--&gt; DP Monitor

Mac --mDP 1.2a--&gt; DP Monitor


But you can't do this:

Mac --mDP 1.2a mst--&gt; DisplayPort Monitor --mDP 1.2a mst--&gt; DisplayPort Monitor


through a Thunderbolt port and configure them both as extended desktops (the second monitor will mirror the first one).
You should be able to and it may be a limitation of the system you are on or any intermediary devices.

Here's my setup:  Dell Precision M3800 with nvidia Quadro K1100m graphics.  Connected out of the Thunderbolt port (by a mDisplay port cable) to a Dell U3014 monitor which is connected to another U3014 (by a full size Display port cable).  I had to enable DP1.2 on the monitor in the settings for it to work.

I'm looking to see if I can use the Elgato Thunderbolt 2 Dock in between my laptop and the first display to utilize Network, USB, etc... in addition to dual displays.