There never was an official explanation for why that box set lacked a 5.1 mix, but I feel fairly certain that if there already was a 5.1 mix done for "Tango", it would have been released in the box set, so unlike "Fleetwood Mac" and "Tusk", I don't think they are sitting on an unreleased 5.1 mix for "Tango".
Maybe they lacked all the multitrack tapes, a suitable and willing surround mixing engineer, or the time and expense to get it done in time for their desired release date.
Either way, unlike "Tango", we knew there was a 5.1 mix of both "Tusk" and "Fleetwood Mac" in the vaults, so the fact that we will now have both of them within the first few months of 2018 makes me very excited indeed!