I feel their popularity is caused by the games themselves. It started with all the developer of agar.io, who found some really good mechanics that became immensely popular. The subsequent io games unblocked followed the identical key principles and thus also became popular.
On the surface level, what these games have in common is the fact that player starts within a weak/fairly powerless position. They can be easily demolished by many people other players, but if they play it safe, it isn’t too difficult to advance. In every three games, the larger/longer/higher ranked players pose a greater threat than their newer counterparts. Importantly though, small players may take on the larger players with smart strategies. In agar.io, viruses may be split and directed at the larger players. In slither.io, any player can definitely defeat a high player if they’re fortunate. And in diep.io players can team together to take on the leaders. So through the perspective of your new player, after some time and energy they too can get to the top. This really is alluring.
And I think this principle doesn’t just pertain to the players themselves. When agar.io first was published, anyone felt that they had a chance to rise to the top. Youtubers started making videos in regards to the game, after which other youtubers believed they too could become popular by making agar.io videos. Then, other developers felt a similar compulsion to either clone or make another kind of .io game that might top charts. Thus .io games have grown to be a phenomenon, which is why rim sim continue to pop up and grow popular.
Another thing the games share is definitely a high depth to complexity ratio. That is to say, there are lots of possible outcomes within the game because of very simple controls and rulesets. In agar.io, all you need to know is that you move using the mouse, split with space, and eject mass with w. With this particular knowledge, start to try out. Needless to say there are several other activities to find out in the game, like the truth that viruses can pop you if you’re large enough or special dexipky52 to become a good player, but what is important is the fact that this data isn’t necessary at the beginning and isn’t ever even proven to the participant in the form of a tutorial. The ball player is left to his/her devices and can discover these details with out everything piled on in the beginning. The controls for wormaxio at rimsim are even more simple. Mouse to maneuver and space to enhance. But from all of these simple mechanics plenty of interesting scenarios pan out.