Because it isn't really special! Is that so hard to see? A heavy hitter having a heavy hitting smash attack is about as obvious and natural design as they come. Why would you even list that as a special attribute? Doesn't really help their seeming poor balance between each other. There is no real justification for saying it's some big special perk when it's something you'd expect from a heavy hitter. You don't look at Ike, Ganondorf, Bowser, Incineroar, Ridley, etc.'s forward smashes and go, "Wow, that really is a special attribute they should dedicate presentation time to." The counter-arguments don't resolve that issue. A forward smash like this isn't really special, yet they used it as her gimmick for some reason. No, being strong isn't a special gimmick, especially for a smash attack of all things which are generally meant to be stronger. It's not even the strongest. It may have a combination of attributes that no others have, but it's not the only one that does this.
In what conceivable way is her forward smash special? The most anyone can actually seem to answer is that being strong is her thing while seemingly ignoring the fact that being fast is Mythra's thing, but none of her standard attacks are considered special. At least admit that.