Why wouldn't someone share his alcoholism and depression?

Shame. Privacy. He didn’t want your concern or sympathy.

He was under no obligation to do so. Private is private.

Its easier to deal with other peoples pain than your own. I’m glad he is getting treatment and hope you all can help him as much as he helped you.

Talking to you and others about his alcoholism would mean admitting to himself that he has a problem.

Denial is deep and strong: I know people who have been hospitalized with cirrhosis that claim they are not alcoholics and do not have a drinking problem. Denial is a significant symptom of the disease.

Alcoholics often “self medicate” depression with alcohol – people with depression drink to cope with their symptoms but the alcohol ends up making them feel even more depressed and they tend to drink more.

There is bias against workers with medical issues, even more so if mental or chemical dependency. Even when inaccurate or irrelevant, it colors perceptions of competence, confidence, and strength of character.

Some people are private individuals, especially when it comes to what they share at work. I’m friendly with most of my coworkers, but I talk very little about my private life at work.

You will often find that people who are eager to help others with their problems are just running from their own.

He could be feeling shame that he didn’t reach out sooner. Maybe he was trying to tackle the issue on its until it got the best of him.

Maybe he felt that revalation would damage future career opportunities.
It is possible management might pick another “more stable” person for the next promotion.