Please explain it in simple terms. Why was THIS asteroid chosen among all the thousand of asteroids?
This selection was made in 2008, when only 7000 near Earth objects were known. This asteroid was chosen for several reasons:
It's easy to get to.
This is the most important concern. Most of our interplanetary missions never come back to Earth. One of the key goals of OSIRIS-REx is to return a sample of the asteroid to Earth. The vehicle has to carry the fuel needed to return home. That fuel that returns the vehicle to Earth is just extra payload as far as the outgoing rocket is concerned. Thanks to the tyranny of the rocket equation, that extra payload mass means even more fuel is needed to send the vehicle to the asteroid. Of the 7000 known near-Earth objects in 2008, only 192 had the right orbit that made a sample return mission possible.
It's the right size.
Smaller asteroids can be spun up by the Yarkovsky effect. A rapidly spinning asteroid would make landing rather difficult. Having the right size cut the list of 192 down to 26.
It's the right type.
A number of scientific and engineering principles dictated that the target asteroid be a primitive carbonaceous asteroid. Of the 26 that passed the first two filters, 14 were of an unknown type (so they were out) and 7 were of a known but undesirable type. That cut the list to 5.
It's potentially dangerous.
101955 Bennu has a 1/1800 chance of hitting the Earth in 2170 and a 1/1000 chance of hitting us in 2182. Sending a vehicle to that asteroid will accomplish two ends: We'll get a much better idea of its current orbit, and hopefully we'll gain a better understanding of the Yarkovsky effect and its second order cousin, the YORP effect. Those effects result from warming caused by the Sun and cooling at night. This makes the asteroid's orbit change, which in turn makes it hard to predict asteroid orbits over a long span of time. Of the five remaining candidates, this was the only dangerous one, and this is at least potentially a very dangerous asteroid.