Why SoftBank and not others?
"Perhaps the biggest surprise about the sale of ARM is the fact that Intel is not the buyer. The chip manufacturing giant had long been rumored as a potential buyer for ARM, which has chip designs that have replaced Intel's in millions of smartphones and tablets around the world."
There has been specualtion about other possible buyers for Arm for a few years, since the IP company has become the de facto standard in the mobile processor business. SoftBank, in fact, is the only one big enough to do it and keep regulators happy, especially the EU Commission and the FTC. If the buyer were to be Intel, Apple or Samsung, the deal would probably fail the necessary approval process.
That being said, it is still early to say that the deal is final. Arm shareholders, the British courts and the EU Commission will have to approve the purchase. And there is still the possbility of someone else offering more.