'It slipped my mind', why not say 'It slipped from my mind'?

[Iwanna]

Is the preposition "from" necessary here or not? why?

[Lost_Soul]

Well, the usual collocation is to slip somebody's memory/mind. You cannot add "from" to this - it would sound weird, I guess.

[Yankee]

.
"It slipped my mind" is the standard, idiomatic wording.
The word 'from' is neither necessary nor is it used in this expression.

(Look at definition #56 here.)
.

[Iwanna]

many thanks