The largest nickel reserves known are in very unpleasant places in the upper regions of Russia. My point is that recovering the same minerals and metals right here on earth is far more efficient than asteroid wrangling.
A lot of terrestrial resources have political risk.
Russia, Congo, South Africa, etc.
As for “Why asteroids?” Its all about Delta Vee; that is, the change in velocity required. In order to get to the Moon’s surface from Lunar orbit you need to change velocity 2.6 Km/sec (IIRC). In order to get back to Lunar orbit you must again change by 2.6 Km/sec.
There are lots of asteroids where you can use gravity “slingshots” and flybys of Luna, Earth, Venus, et al to get Delta Vee requirements down to .5 Km/sec or less.
Remember, doubling Delta Vee takes four times as much fuel because of MV2.
Also, mining asteroids is much less controversial than claiming a piece of Luna. You perform a public service by removing a rock that could hit Earth in the future.
As for ownership, was there any question of who owned the rocks Apollo returned from Luna? If you bag it, you OWN it in space.
Add in a Space Elevator to geosynchronous orbit and beyond (now possible with carbon nanotubes 600X strength/wt of steel), and you have a real High Frontier!
minerals and metals right here on earth
But not as pure which makes all the difference.