I don't think it's much more complicated than that. If you own a "rent-stabilized" apartment there's going to be a very low limit to the amount of money you're willing to spend to keep it habitable. If it needs a lot of money to make it legal to rent out, you're just going to leave it vacant. You don't really have a choice.

> And the reasons why they are vacant is much more complex "because city regulations on rents make them unprofitable to rent out".

Is it actually more complex than that? If the regulation that makes it unprofitable to rent them out is rent control, that's still an accurate statement of why they're not being rented out.