Why did opponents of slavery's focus on ending the slave trade rather than slavery itself

Because the slave trade was giving more slaves to people

It’s easier to stop a moving car than to destroy the car entirely. Similar to the other answer here - abolishing slavery was never thought of as an “easy as 1, 2, 3” kind of task. The southern states heavily relied on the institution of slavery within its economic system. If slavery were abolished the next day, the economy would’ve collapsed on itself, which would hurt both the white plantation owners as well as their slaves. Ending the slave trade is a much more reasonable and realistic objective. Moreover, as the other answer mentioned, slaves were treated as property and brought to the states like shipments. Ending the slave trade would at least end the arriving of new slaves.