It's because a block reward (a coinbase tx) can be used as deposit directly. Or rather, a deposit tx can spend the coinbase tx in the same block. If the deposit change could be spent straight away, you would be able to get around the coinbase maturity rule by directly spending the coinbase tx with a deposit tx and sending everything as change. That's the simple reason the restriction is there.

You could of course argue that the restriction only should be there if the deposit tx spends a coinbase tx. To simplify the validation of blocks, however, the rule is applied to all deposit txs.