It was such a massive breach and it was surprising how little press it got. The "Russians" hacking the election with flaky evidence has gotten magnitudes more of coverage.
And here is detailed information about current and present government employees, many in the intelligence agencies, tens of millions of people total probably. Including biometric data like fingerprints, deeply personal stuff like past transgressions, addictions, financial trouble, health information -- these are things that could be used to approach or entrap someone in a blackmail scheme. And comparatively it was barely a blip in the news.
CIA if I remember correctly was the only not affected. But it is not unheard of people moving between agencies so there is still risk there.
OPM asks applicants very personal questions to see if they are being honest. The leak included information on government employee's that have clearance about infidelity, drug use, debt, and other sensitive topics for people working in very sensitive positions. Also more benign stuff like every place they have ever lived and people they had contact with while living in those places. That information could be used in a lot of ways.