In the blog post about the addition of Duos, community manager David explained why they have decided to add Duos as a permanent mod, but will not do the same for Solos. He says that Duos had a positive effect on overall health of the game "in regards to queue times and gameplay systems in addition to reinforcing team play."
"Now, what about Solos? When we introduced Solos as a limited-time mode last year we saw it actually negatively impacted the game, especially when it came to new player retention," David says. "Weâ€™ve also purposely designed Legends and their abilities to compliment teamplay and squad composition, but when played Solo some Legend abilities become useless. These are just some of the reasons we decided not to include Solos in todayâ€™s update."
But don't worry, they aren't completely getting rid of Solo, they will just not add it as a permanent mode. He explains that they are still exploring ways to allow a Solo experience, and they will talk about it when they have something ready.
Duos mode came today along with The Old Ways Lore Event, which offers new town takeover Bloodhound's Trials, new daily challenges and a plethora of new cosmetics.