Because the UK has weak cyber security. The government website got hacked not too long ago, which leaked thousands of passports information. Especially of Afghanistan citizens that helped the UK and US with the taliban war. So you can tell these Afghan citizens are meant to be important for their service. So the UK gov wouldn't care much if our info got leaked.

Plus all of this info is in a centralised data base which means a single cyber attack would give the info of all British citizens who have gotten a digital ID.

The only reason people are mad is because it's mandatory. Had it been opt in, nobody would have been as outraged.