That' s because there's no reason to use Elena over Chun Li in Third Strike.
I don't think they're that similar. Chun is only good when she gets two bars of meter, Elena is good from the get go. Elena has a way better mixup game, Chun has like no mixup game until you can kara throw. Elena can't force you to block down the same way Chun can because her down moves are kind of bad and can't be confirmed into super. I've heard a lot of people compare them and the only resemblance I see is that both rely on pokes until they get meter, and they both have really strong moves that deal with a lot of situations (back fierce for Chun and back roundhouse for Elena). Aside from that I think they're pretty different. Elena's worse than Chun, but so is almost all the cast.