Why Huck wrote where the money was instead of telling it to Mary Jane?

Huck didn't want her thinking about where the money was..... he didn't want to distress her. It was easier to write in down and have her look later.

I didn't want to set her to thinking about her troubles again; and I couldn't seem to get my mouth to tell her what would make her see that corpse laying in the coffin with that bag of money on his stomach. So for a minute I didn't say nothing; then I says:
"I'd ruther not TELL you where I put it, Miss Mary Jane, if you don't mind letting me off; but I'll write it for you on a piece of paper, and you can read it along the road to Mr. Lothrop's, if you want to. Do you reckon that 'll do?"