Mama, listen to what Ladyscribbler said:
"His primary interest is in maintaining the staus quo because it gets all of his needs met and protects his addiction at the expense of you and the kids."
It is exactly this, and nothing else. I see now that my STBX wasn't really afraid of losing me- he was afraid of losing his lifestyle. He knew that random drug tests were coming his way, and he didn't want that! He also knew child support would be considerably more if I got full custody. That's drug money- he needs that! And I think, more than anything, I was a really good chump. He knew it could take time to find someone as naive as me, and even though I'm sure he had a backup supply lined up already, they hadn't been thoroughly vetted yet. He was still testing the limits of what they'd put up with.
Please be strong, Mama. My prayers are with you.