Because they don't care if cotton is or is not on the seal. They DO care if there is a cross on the seal or not. They are trying to head off an ACLU lawsuit. But if they just go ahead and say they are trying to take the cross off, there will be an uproar from everyone concerned. Claiming it is about "cotton" is a way to eliminate the cross without pissing off a bunch of alumni. The stealth approach works.
Just think: how many seals have had to have their crosses removed in the last two years? Several cities and Los Angeles County are the most well-known. How many have had controversy over depictions of "cotton"? None. They are trying to stave off an ACLU lawsuit.