The pre-paid card costs a lot to run - it uses a third party processor to deal with all the transactions. That means monzo has no control over how that works and when things go wrong (as has happened a fair few times) it’s been the card processor that has suffered an outage. It’s unreliable, expensive and not monzo’s product effectively.
You can still use a current account in exactly the same way - transfer a pot of money into your CA and spend it as you would do on the pre-pay. It works exactly the same, and if you don’t ask for an overdraft, you won’t get one which means you also can’t overspend.