Some transactions are not applied to invoices
Some transactions are applied as credits, and then applied to invoices via the cron, so the items being paid are not known at the time of the transaction.
A transaction might be applied to 1 or more invoices, including a partial payment to multiple invoices
Do you want the objects for all invoices the transaction was applied to to be available to the email template as tags?
Let's say someone partial pays an invoice. There's no way to know for which items they intended to pay, so listing all items on the invoice in the email may be inaccurate and/or misleading.