How the Grinch Stole Christmas was on t.v. tonight. (Best Christmas cartoon of all time). It occured to me that I have no idea why little Cindy Lou Who is drawn with antennae on top of her head. The Whos appear to be some sort of humanoid species - certainly not insects, and most of the Whos don’t have antennae. (It’s hard to tell because a couple other Whos have things sticking out of their heads that are either antennae or hairs.) Is Cindy Lou the only Who with antennae? - and why does she have them?
Because that’s how Dr. Seuss drew her.
From what I remember, he drew Whos down in Whoville both with and without antennae. I can’t find any major excerpts online to show you, though…
Why antennas for a Who?
Why, all larval Whos do.
Because it’s cute.
Some of the Whos were born with antennae
which stuck out from their heads like little penne.
Others however, had heads that were bare.
There was nothing on them, nothing but hair.
But did one group of Whos treat the other with shame?
Did they laugh, did they point, did they call dirty names?
Did the bareheaded Whos put the rest on the spot?
Did they? Huh? They most certainly did NOT!
They met, they joked, they shared in their feasts
of puddings, and porridges, of fruits and roast beast.
Their kids played together, they lived side by side
They looked at their differences with love and with pride.
And that’s a lession we can learn for this season
Our differences don’t matter, whatever the reason
So don’t listen to those people who come out and say,
“Don’t talk to him! He’s straight, or he’s gay.”
“He’s black or he’s white, or some shade of brown,
He was born someplace far out of town.”
“He’s an atheist, a Muslim, a Christian, a Jew.
He’s ugly to look at, and handicapped too.”
Don’t listen to them! Don’t listen, I say!
When they start to talk, just walk away."
For you’ve learned what the Who’s already know
A person’s a person, if he’s fast or if he’s slow
And whether he has an antenna or a hat,
A person’s a person, still, for all that.
That was Amazing Captain. I printed it out and posted it on my wall.
Well, I certainly can’t argue with that logic. 
The television special wasn’t drawn by Dr. Seuss. It was drawn (“directed”) by Chuck Jones. Jones modeled the look after Geisel’s original book, but it wasn’t even really much more than an approximation.
Therefore, Jones made Cindy Lou Who with antennae because Geisel did. I assumed you were only asking about the TV special.
I definitely didn’t mean to just stick a tautology out there and expect it to stand. 
Because she doesn’t want to pay for cable.
Jones worked pretty closely with Geisel on it though, didn’t he? They both have “producer” credit in the cartoon. It’s not like that bastardization with Jim Carey.
Oh, I’m not saying that Jones took off on his own through uncharted wilderness or anything. I’m just saying that the TV special has Jones fingerprints all over it and isn’t a direct translation of Geisel’s art to animation.