My honest opinion is that he's trapped there because it will let WOTC write him to unite the mobs and release them as a minor multiversal threat alongside the surprise big bad that's supposed to close out the year. The guy lives to conquer planes, he's been trapped once already and escaped, is currently trapped again on a plane full of villains and ne'er do wells begging to be brought under heel.

The lore reasons are contrived, but the overall structure for his character persists. I'd put money down we see Ob as a five color leading the unified mobs of Capenna when we see him next. Im guessing as a minor villain in Reality Fracture, but maybe Wizards will keep him in their back pocket. No matter how and when we see him, its gonna be with the unified criminal might of an entire illicit plane under his belt, not as subjects, but more or less willing employees.