Why can I only do one ‘attach’ transaction per block?

If a user submits two attaches in the same block, there is a risk that they will try to attach more than their total asset balance for a given asset, or that they will attach multiple assets to the same UTXO. The former will result in the second-confirmed transaction to be rejected and the former will result in the user accidentally listing more of an asset than they intended. To prevent this, Horizon Market does not allow users to attach more than one asset per block.