Because bonds involve interest. Per Summa Theologica:
> To take usury for money lent is unjust in itself, because this is to sell what does not exist, and this evidently leads to inequality which is contrary to justice.
https://www.newadvent.org/summa/3078.htm
…Aquinas expands the analysis but it is relatively straightforward: all interest is usury.
Personally, I find it helpful to imagine two hypothetical persons representing the entire economy, one the creditor who is lending and two the borrower who is taking on the loan. In this ultra simple closed model with a fixed quantity of money, the former is in effect asking for more units of money than actually exist in the whole system. When the loan comes due the borrower owes a sum that cannot be paid in full from the circulating medium itself. Settlement then requires either default, the creditor forgiving the excess, or the transfer of real goods and property to make up the difference. Scaled up, that same pressure (the continuous generation of monetary claims that exceed the existing stock of money) is what I suspect drives a good deal of the subtle and overt strain on families and communities that people so often complain of in the West and in modern growth-oriented capital societies.
This definition of usury differs from the modern loophole-definition: that interest bearing loans are only usury when the rates cross some nebulous abusive threshold. In the above Thomistic interpretation, all interest is socially problematic and disfavored. Judaism holds to a similar prohibition on interest when loans are made between Jews. Islam likewise prohibit usury even more broadly. Despite the injunction against usury in the Middle Ages Christendom and the enduring prohibitions of usury in other faiths, there are many modern Catholics and Protestants who will favor the modern interpretation over Thomas’ understanding; I’m just not one of them.