I would wager, to make a point to certain people.

I think Sjors wrote about this on delving

Citrea does use witness data for normal operation.

The reason Citrea can't use inscriptions for fraud challenges is that the output is later spent during the withdrawal. So when the UTXO is created we don't know the data that will be committed. This is because we need the Schnorr signature (which spends the corresponding output) to commit to the longest chain proof.