The answer is: “Because, as I said, I want to transform this country. What you rubes don’t understand is I intend to transform it into a turd-world hellhole.”
If so, it may not be a such a great place for him to live in a few years.
He will announce it just after (almost immediately after) the grand jury in Ferguson makes their decision public. He is in hopes that the media will spend all their time covering the riots, burnings, rapes, murders and lootings instead of the illegal amnesty issue. Like a thief in the night he will give the executive order.