I warn you that this section gets a little bit speculative. I can't pretend to know the full reason why the Handys leave so many of their properties vacant for so long, and honestly, for every property, it's probably a slightly different story. What I can tell you is that there are very strong financial incentives for leaving properties vacant when you are irresponsible with your finances, and this can often lead to irresponsible landlords trying to socialize that cost on everybody else.
The most likely thing the Handys are doing, particularly with some of their newer purchases like Nectars, is a scheme with the bank called "extend and pretend". For a very detailed and thorough explanation, please read this article by Strong Towns.
Summed up, the Handys took out a bad loan and told the bank the property was worth more than it actually is. The Handys can't lower the rent, because if they do, it would be proving that they took out a bad loan and that the property is worth less than they claimed, which would force the bank to foreclose on the property. They also probably took out a loan so bad (based on their asking lease price), that even if someone wanted to outright buy the building, they can't possibly offer enough to get the Handys out from under the bad loan. Again, please read the Strong Towns article, and then come back here for some numbers!
Assuming that the Handys are doing "extend and pretend" with Nectars, they likely took out a loan from the bank claiming the building is worth about $5,040,000. We know this because of the rate they are asking for rent. Now, if they lower the asking rent, even by a little - say they bring it down to $10k/month, which would still be egregious- that would lower the value of the overall building all the way to $2,400,000. That's a more than $2.5M difference! So if the banks foreclose on the loan at that price, the Handys are on the hook for $2.5M that they don't have - which means they will never lower the rent, and the building will stay vacant. They will "extend" the loan and "pretend" that someone will eventually pay the $21k to rent it.
If someone wanted to buy the building from the Handys' outright, in order for the Handys to get out from under their bad loan, they would need to offer at least the $5M they claimed the building was worth. Nobody is going to do that, since its assessed value is about $3M less than that. So we'll extend and pretend till we are dead.