Of course, that is assuming Microsoft's "It's complicated" is referencing it being a technically complicated reason.
Or, it could be "complicated" inasmuch as MS wants greater control over distribution methods and timings, and only after making the 10586 iso available did they realize that doing so somehow negatively impacted their ability to control/monitor installs in a way that they believe affects/has potential to affect their bottom line.
It could also be "complicated" because it might be the case that the alternative authentication options available with the 10586, such as doing a clean install with a Windows 7/8/8.1 key opened the door for pirated versions to upgrade to 10 "authentically." Regardless of the hype before the release, MS has no interest in giving Windows 10 away for free to the world's pirates. Already many have claimed that using the regular upgrade feature from a pirated version of 7/8/8.1 leads to an authenticated 10, but doing a clean install of 10 isn't possible, or is more difficult.... maybe the 10586 iso made it easier...
It's complicated to be a little bit truthful, without opening the floodgates to the real reasons why MS don't want all and sundry to clean install at the moment, but still have to allow upgrades.
A similar thing occurred at the end of July too when 10240 went to general availability, and all the activation servers for Insider previews upgrading to 10240 shut down. Also the reason given then was to see how the upgrading from Qualifying OS installations (7 & 8.x) were getting on.
Seems to me that there is just no control that MS can put in place to stop any old OS COA 7 or otherwise obtained non-slp 8.x key activating an install on any old non-qualifying device with the right edition from the ISO and MCT esds or wims that have been available since 10565.
Again, 100% speculation, but that would explain the "activation issues" Microsoft has mentioned without them spilling the details about it.