Slade desperately wants a Robin of his very own
I thought Slade had come a long way from his past of fighting Teens and being a TT's villain. Certainly at the point where he was being considered for the foe in the solo Batman movie but it seems that on the side he gets up to his old tricks.
I don't know why it's so hard for him to move beyond that. Some writers like him in that role or maybe he's just one of the few decent enough baddies for teen heroes to fight. He's just got enough cred, isn't fodder and he isn't so high level he'd need to job for it to be believe.
Also he's a free agent [isn't arching any heavys who all have their own rouges] so he also wound up being available for TT writers to use.
He is morphing into Damian's Archrival. They had a history/dynamic which Priest is keen to explore.
They work well off each other and their relationship has evolved over the years.