I've wondered as well and so I don't have an answer I'm confident in, but I have an intuition about it.
I suspect that it's there to lessen the likelihood that the voting will become politicized. That is, if everyone must vote, then the act of voting doesn't carry any connotations one way or another.
So, you don't want someone abstaining because they're afraid to make their opinion transparent (while it's understandable it generally only furthers the dynamic), so voting isn't a choice. You have to pick someone, BUT you can just say you picked them for "No reason," or, "For the reasons already given." And with that explanation provided to them, I've never had any issues or additional requests to abstain.
I think requiring a vote parallels the principle of a governance meeting in which the agenda-item owner must present some sort of proposal. That is, you must provide a suggestion of some sort. You can't just complain and expect others to fix it for you. Since an elected role is an output of everyone's vote, you need to contribute to that output. And, as I said above, that doesn't mean you must have a strong opinion, you just need an opinion. Just as you must respond in the reaction round and objection round even if just to say, "I don't have anything to say."
Again, I don't feel super-confident in this answer because I've never practiced the process without requiring everyone to vote, so I don't have any first-hand experience with the tension/s that might have led to this.
Now, with that said, here is where it gets tricky for me. And this is kind of along Paul's response. Imagine you leave a governance meeting to go to the bathroom and you miss the processing of an agenda item. You miss everything. So what? That happens. Hopefully, the Facilitator won't follow you in there to see if you have an objection.
Or, you're in a governance meeting, but you just got an important email and decide, "Hey everyone, I'm officially leaving this meeting, but I'll just be sitting here responding to this email." It seems very similar to abstaining from a vote.
In these cases, nothing is GAINED by removing yourself from the process, but I can't say that anything has necessarily been LOST either. Since we can "put on" or "take off" any of our roles in an instant, it's like you just said, "Hey Facilitator, for this one agenda item, I'm not going to be here officially in any of my roles. I'm just going to be sitting here."
With that interpretation, I may prohibit anyone from abstaining, but I can't prohibit them from deciding at that moment, for that item, they are no longer "in the meeting." This is effectively the same as abstaining, but with two caveats.
First, abstaining by removing yourself from the meeting means that neither the Facilitator nor the Secretary could use it as a loophole because they'd still there at least in those roles and therefore would at least need to explicitly hand over authority to someone else to try and use it.
Second, even though you can make that argument and use that interpretation to effectively allow for abstaining, you have to ask, "But why would you need to do that?" Again, in the examples I gave above like going to the bathroom, sure that happens because there are occasions when things actually need your attention or are more important than the meeting itself, so responding to an emergent need makes sense.
But intentionally trying to "leave the meeting" for a particular item seems really strange. Like, the expectations are very low. Don't ask questions. Say, "No reaction," and "no objection." And I imagine most people who have been through a lot of governance meetings have at some point done this exact thing.
So, again, "Why would you need to leave the room/abstain?" The only motivations I can imagine are all based upon misunderstandings of how the process works, or what a vote signifies.
Which is it's probably important to address those misunderstandings by requiring a vote (but allowing or encouraging them to say, "No reason"), rather than enabling the confusion.
With that said, whenever I've Facilitated a client meeting and a vote comes up, I always find it weird to nominate someone since I don't know anything about anybody. And while there are benefits to just following the rules (i.e. I know to just stop at a stoplight regardless of whether or not another car is coming), there are also costs to following rigid rules when there may be better solutions (i.e. maybe that stop sign should be a yield sign instead). The most impactful of which potentially eats away at one's trust in the rules as a whole.
So, if anyone has any suggestions for language changes in the constitution regarding the election process, please share them here or on Github as that might just be the best way to go.