It seems as though everyone said it but Willis. Phill said it in the episode Willis and Arnold went to apply for Welfare and Kimberly said it in Jacksons. Wonder why he didn't say it? But his catch phrases were cool too

A possible reason Willis never said it is because he had his own catchphrase "say what" but Drummond and Kimberly didn't have any catchphrases of their own.

I have to Diagree Kimberly didn't but Drummond did. It's very unnoticalbe but its his catchphase and it's only one word: What?. He always says it really fast therefore you might not catch it. Willis had more then one, he had three but I'll save those for another time

Yeah, I didn't really think of that "what" as being a catchphrase.
What do you think were Willis's other catchphraes?

Oh, and another reason I thought of is since the original populer catchphrase was "What You Talkin Bout Willis" it could ruin the joke a bit if Willis said it.

i think he said "what" the same way in real life. on the E hollywood story, he was saying something and said "what" the same way he did in the sow

I think that the producers and the writers were using good, real world sense. A big brother woudn't be likely to use a little brother's catch phrase. That would be intruding on little brother's territory and maybe just plain not cool for big bro' to be mimmicking little bro'.