Because he's responsible for both companies, and he should spend a majority of time on whichever company needs more of his time (which as you suggested, might be Twitter).
No, for the same reason you don't spend most of your time on your weakest employees or most of your money on your poorest investments. Feed your winners.
You have two choices when it comes to the weakest employee. Either you reach out and spend time to UNDERSTAND how to help this employee, or you can just ask the employee to leave.
A good manager would do the first unless there is absolutely no sign of any improvements. What is the definition of winners anyway? The one that gets the most code done and go home at 3PM? Or the one who is capable of delivering good work AND willing to help others to succeed? For some, bring a down company back on its feet is the most challenging and rewarding project.
Dorsey has to choose. Perhaps he just isn't the right person to lead Twitter. Or maybe he has to give up one child over the other. This isn't the simple "let me sell the bad assets" question. Twitter is still living, and is making money, just down the slipping route for far too long. Snap is in the same boat.
It's hard to let go of the control when you co-founded both and you have visions you really want to make them happen. On the other hand, why overwork? If I were him, I pick one so i can have some breathing room (but I rather quit and spend my day time on non-business stuff like earn another degree or hike around). Just my thought.
