It would be easier for me to explain if I reverse the question
"Why do people think Sheik is better?"
The reason why Sheik is better then Zelda is because in short Sheik is still developing while Zelda is for the most part done. I guess this could be said about a large some of the other characters who are listed higher then Sheik but the difference is that the amount of goodness Zelda and Sheik already had was for the most part even maybe leaning a bit towards Zelda but since Sheik is still getting better there's a very good chance she will surpass Zelda even with her lighting kicks.
IF you ask me Sheik is already better then Zelda by great lengths but that's just my opinion. (haha, I avoided saying the "P" word)
well this is biased.... the truth is, more people are developing shiek's metagame.... it's silly to say Zelda is done developing... she's developing much slower, but who's to know when she's done developing.... in fact... that only makes zelda even MORE the better of the two... that she's less developed and still considered better.
But, if we are going to be honest here, the surest way for Zelda OR shiek to move up in the tier list would be for people to start using Zelda/Shiek.... whether they transform midbattle or just ingame counterpick... it's still a big plus.
Ok... now down to brass tacks.... why is Zelda better?
- Resistant to grab based strategies (falco, ness and ICs really hate this.)
- Shuts down air-to-ground games
- if you can't outcamp her, good luck approaching her
- teleportation makes her DANG hard to edgeguard (though not the hardest to edgehog).... though shiek has about the same plus there.
- smashes are very difficult to dodge... no other smashes really have the duration that hers do.
- practically every move is a kill move
- Din's might be dodgeable... but MAN does it pack a kick for what it is.