why does abigail not want betty to reveal that abigail drank a charm to kill goody proctor?

Simply Abigail needs to save her own behind and quickly. Abigail's best defence for the whole dancing around the forest naked bit is to place blame on Tituba the slave girl. If it should come out that she drank the "evil curse Goody Proctor drink" it would be known that Tituba was not solely responsible. It would also come out that some sinful stuff was going on in the Proctor household and Goody was shut out of the fun! Abigail is pretty firm on all the other girls shutting up about the whole drink thing. Check Abigail's creepy threat out,

"Let either of you breathe a word, or the edge of a word, about the other things, and I will come to you in the black of some terrible night and I will bring a pointy reckoning that will shudder you..."

That's scary!