One of the reason, because of the mobile game colabs / tie-ins.
Just like how Shadow appearing in other mobile games (Angry Bird 2, Ragnarok Online) for "Fearless" event colab, Silver would be to.
And in past mobile games, somehow Silver was the most popular character because how OP he always was (and also always hard to get).
Silver's always treated as SUper rare legendary character and designed to be more powerful than Sonic, Shadow, Knuckles, Tails, or Amy.
As example, in SEGA Heroes and Monster Super League, Silver wasa must-have S-Tier character while others were A/B/C Tier.
I don't know why other developers love to make him the most powerful one.
May be they thought his power (kinetic, flying) was the most interesting.
You think if Silver gets his own Generations, Mephiles will have another chance?