Because the nature of this acquisition is completely different. SoftBank left Arm mostly alone; i.e. brand survived, all offices survived, etc. If this deal goes through, Arm will get fully integrated into Nvidia. There won't be an "Arm" anymore.
But the British government has experience, from the Kraft purchase of Cadbury, that even if the buyers promise to keep jobs in the UK and product quality high, once the deal goes through they can renege on those promises instantly with no negative consequences.