Direct Debit is a pull based system, best suited for collecting lower value transactions. Larger value payments carry with them an increased risk due to the ability for customers to chargeback payments collected in error under the Direct Debit Guarantee.
In consideration of this, we have a transaction limit in place to protect both merchants and GoCardless from potential large losses. You can see an overview of all transaction limits here.
If you would like to read more about the Direct Debit Guarantee, you can do so here.