Have you seen the estimated values of ores in a decent asteroid?
I think it's technology and up front financing. Well, and there are probably space treaties that are problematic.
I'd drop an ion drive on the asteroid and nudge it towards the earth (but not AT it), and once it's in a stable near orbit, figure out what to do with a much faster communication loop with remotes/robots:
process in orbit, drop parts down to earth, or some combination.
Yes I have seen the "estimated values" of asteroids. They're not that meaningful as they're only estimates of reserves with no regard for production. Prices of platinum group metals are what they are because known reserves are limited and production even more limited. If you suddenly increase the supply of PGMs they're now much less valuable and their price drops. Extra supply doesn't automatically create extra demand.
But that's besides the physics problems. It's not in any way simple to attach an ion rocket to an asteroid and change its orbit. Today's best ion engines used on probes deliver fractions of a Newton of thrust and requires several kilowatts of power to do so. That's just to accelerate a relatively small spacecraft (Dawn is a bit under 2 tons).
Adjusting the orbit of an asteroid of a non-trivial size with an ion engine would take orders of magnitude more fuel, power, and time than the Dawn probe. Even if by some magic you managed to mine the fuel from the asteroid itself, a technological feat unto itself, you still need power and time.
Power and time are doubly impacted because most asteroids rotate. Unless that axis of rotation is perfectly aligned with the desired trajectory you can only apply thrust for at best half the asteroid's rotation. Rotating is problematic for power generation. If the asteroid miner is solar powered it's panels would be in shadow half the asteroid's rotation so it needs extra mass and complexity for power storage. If it's nuclear powered it's radiators are in continual sunlight for half the asteroid's rotation so additional mass and complexity is needed to rotate them parallel to the bearing of the sun to remain effective.
Even with all that it could take centuries to move an asteroid of any significant size with ion engines. Even if it only took decades that is still a huge initial outlay with zero payback for decades. The only way that's even remotely sane is if you spammed every asteroid with probes to assess their composition and knew you were pulling in the Comstock Lode. But then you depressed the price of all the material and the whole effort barely pays for itself.
When the Spanish extracted all the silver and gold they could from South America, it caused a dramatic reduction in the value of silver and gold in Europe.
I.e. dropping a huge chunk of rare Earths onto Earth will make them not-so-rare and prices will collapse.
I have seen people imagine what those ores are worth when they pretend that they are somehow magically transported to the surface of the earth, but I have never seen these same people budget the cost of the energy to retrieve, return, and land those same resources. If your value calculation did not go negative there then you are doing it wrong.
That's the interesting and confounding part of it. Going up to get them is expensive, sending them back rather less so.
(the major cost of doing anything in space is the earth surface to low earth orbit tax, which is going to be over half your cost. If you can avoid that tax, suddenly a lot of things become very interesting)
I think it's technology and up front financing. Well, and there are probably space treaties that are problematic.
I'd drop an ion drive on the asteroid and nudge it towards the earth (but not AT it), and once it's in a stable near orbit, figure out what to do with a much faster communication loop with remotes/robots:
process in orbit, drop parts down to earth, or some combination.
100% chance SpaceX is considering this long-term.