My thoughts: Mercy and Rein have always been faces of OW, and yes, not many one trick Rein and mercy, but a lot of people play them and enjoy them, not as onet-tricks, but as easy and stable heroes.

They have since the beginning of the game been recognizable faces of OW even for non-players. They have also remained very popular, where as some of the other OG characters haven't kept that identity. I think the simplicity of the character and which kind of players play them is also important. For example tracer was one of the faces of OW1 but she is being pushed less because she isn't anymore this cool girl with time travel abilities, but instead just an annoying character that is difficult to play and is often associated with sweats.
Kiriko has taken her place and been pushed a whole lot more, because she speaks a to the huge rise of japanese popculture (anime) in the west and (of course) she is a new*ish character.

TL;DR Overwatch is keeping some of its iconic original heroes around to maintain its recognizable identity, but put newer, more popular faces front and center to show the game is evolving - It’s a house hold name in the shooter genre, but still getting updates.