It's not that the ABC didn't think about it, they traded the rights to BBC in exchange for production funding. The ABC typically only fund up to 30% of an externally produced show, they can't afford to fund every show at 100%. There was no way to know that Bluey would be more successful than any other kids show they fund - especially when a full production run couldn't be made without additional funding from the BBC. That funding came with the condition that BBC Studios would over the global distribution and merchandising rights. The ABC doesn't have the capability to manage global distribution or merchandising, so Ludo preferred this arrangement, and without it Bluey would likely never have been made at all. The ABC can't spend tens of millions of dollars of taxpayer money gambling on every kids show becoming a global hit.