The rarity of sub-rare mana fixing is largely determined by how important access to mana fixing is in the limited format. Khans of Tarkir and Return to Ravnica blocks were multi-color blocks where it was common to play three colors or more. One of Mark Rosewater's design mantras is "If your theme isn't at common then it isn't your theme", and that rings true here. The Guildgates and Khans of Tarkir taplands were at Common because it was necessary for deck building with the multicolor themes of those blocks. Without easy access to mana fixing, the themes would not have played out well in limited formats. For Battle For Zendikar and Shadows Over Innistrad blocks, we've returned to the two-color paradigm where people are mostly playing two colors at most, only occasionally splashing for a third color. When we eventually visit another multicolor-themed world such as Alara or Ravnica again, it's all but assured that we'll once again be back to taplands at the Common rarity.
(Also known as Xenphire)