There are too many posts about this and so far nobody I've seen has nailed it. TL;DR - The reason is because Bungie didn't create anything compelling in the primary category for year 2. They removed elemental burns from raid and ToO primaries and didn't put together any compelling mix of perks that made them feel really satisfying to use. In year 1 we had Vision of Confluence, Fatebringer, Fang of Ir Yut and depending on preference, a few others that were more beneficial for PvE activities than Mida. In PvP of course, the Suros/Thorn/TLW/Hawkmoon/Pulse meta's were all more attractive, but MIDA could still compete even then. Mida was my very first exotic. I used it for everything when I first started, it had a great balance between fire rate and impact. I felt like I was accomplishing something when I would shoot enemies with it. I could kill lower level mobs fast due to the fire rate, I could watch my six due to third eye (this was especially important before I learned the AI spawn patterns and locations), I could stagger larger enemies (Captains, Knights and Centurions) with every shot, it made me move faster, had good range and a good sized magazine. Then when I finally ventured into PvP I discovered it was good there as well. I was terrible with hand cannons, didn't like auto rifles, pulse rifles felt weak in the beginning and ran out of ammo too quickly that left me with scouts and in PvP nothing felt better than Mida for me. Then when I was introduced to VoG by a good friend and got my first Fatebringer and eventually vision of Confluence, I found they were more beneficial for taking out major Fallen, Hive and Cabal shields quickly, add firefly/full auto into the mix and I had a compelling reason to use something other than MIDA. Additionally the weekly Nightfall and Heroic strikes gave me a reason to favor elemental primary's over kinetic damage. When TBD hit I added Fang of Ir Yut and Word of Crota to my go to primary list. So MIDA never became the go to primary for every aspect of the game for most people. There were some in PvP like myself, but even then sometimes it became necessary to use Thorn, Three Little Words, The Messenger or even Red Death depending on your opponents strategy. Are some of the other weapon changes a contributing factor? Sure to some extent they are, but Pulse rifles are still very effective in crucible, hand cannons are on the rise again and autos just got a tiny buff and I'm seeing an increase in their use as well. But the primary reason for Mida's recent success is because Bungie has removed the emphasis they once had on primary weapons. Just my two cents.