Why Stevenson, the author, keeps Hyde’s appearance a secret?

We actually do find out what Hyde looks like. I'm unsure what you mean.

I love the article cited below on the victorianweb....... it's a great look at Stevenson's work..... and how he proceeded to develop the dual personalities in the novel.

Okay, I suppose it is not specific,

"He is not easy to describe. There is something wrong with his appearance; something displeasing, something downright detestable. I never saw a man I so disliked, and yet I scarce know why. He must be deformed somewhere; he gives a strong feeling of deformity, although I couldn't specify the point. He's an extraordinary-looking man, and yet I really can name nothing out of the way. No, sir; I can make no hand of it; I can't describe him. And it's not want of memory; for I declare I can see him this moment." (p.15)

I always thought he looked troll-like but I suppose Stevenson knew that the scary details we make in our heads are way more terrifying than any he could give us.

He doesn't keep it a secret, all the characters assert that Hyde is deformed, ugly and inspires an immediate revulsion yet no one can say why. He is not a natural being which is why he looks like that, but also as a reflection of him, he is clearly an evil character.

Mr.Hyde is always hiding because his name is Hyde.