Mostly because they don’t think it would be necessary to pay fair market value for those guns.
Australia bought back 660,959 firearms at a cost of some $500 million (AUS $). Or roughly $740 (AUS)/firearm. That’d be about $550 (US)/firearm.
And Australia only collected between 20% and 40% of their guns, as well.
So then we get into a kind of a supply/demand price point analysis, on top of what was already done. I’ll yield $550 a firearm, and drop the reduction from 50% to 30%, and you have less money and less lives saved, and both the numerator and the denominator go down. Etc.