We reported in the past about events in Apex Legends where Duos and Solos modes were available temporarily, and we suspected at the time if they were successful, they might make it as a permanent feature.
While Duos mode is now going to make it's way to the full time roster of game modes, it seems Solo won't be getting the same treatment, with Electronic Arts stating the mode simply didn't work witht he game's co-op gameplay mechanics.
It makes sense really, as the game does heavily rely on co-operative gameplay. Duos mode had a positive effect though, with EA stating that it cut down queue times and improved teamplay among gamers, which is exactly what you want in a game like Apex Legends.
EA's community manager David stated that;
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. We’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 today’s update.
Players also apparently had issues with players in solo actually co-operating together in the mode, which put them at an unfair advantage. Friends typically ended up in the same game when queuing at the same time, and many gamers took advantage of this.
The mode may still reappear in a different format, though, with David going on to say that the developers are still "exploring ways to allow a Solo experience", but it'll likely take a different format.