It's just a fact of life. Some things go unnoticed, whereas things that deserve no more than a second glance get noticed heavily. The imbalance in votes for simple, obvious one-liner answers vs. well thought out, detailed and usually late answers has been noted by many, but you can't expect everyone to have seen every single question/answer to imply that the answer is being deliberately ignored — luck and timing plays a huge role in votes.
As for this one, I don't know — maybe it was posted when most people were asleep and then got buried under a new flurry of answers? Maybe there was heavy activity/editing which put a lot of posts on the front page? Perhaps since the top answer had 14 votes and accepted status, many didn't think that answers further down were worth reading? Plenty of reasons. I'll give you this as empirical evidence — I'm a prolific voter on this site, and yet I hadn't seen that answer, although I had read and upvoted other answers there and the question. I've seen it now and upvoted it, but sometimes, things just go under the radar.
However, the problem that I see with actively soliciting for votes like you do (for self or for others) is that people start to subconsciously zone out the requests — at least, I do when it becomes a pattern. Further still, you're creating an invisible "standardized" rating system, by which answers should get a certain "deserved" votes. Where would you like the votes to stop? At what point would you say this has had "enough"? Is this more about the accepted answer having twice as many than this answer (i.e., poor in comparison), rather than having low votes by itself (i.e., poor in absolute terms)? What would you do if something goes viral like Waldo? Ask users to remove votes/downvote it? I think this is a dangerous direction to proceed in.
A partial "solution" for these would be to get the community more involved in voting and editing — very few users actively do both. Editing such low voted answers by fixing minor mistakes or perhaps adding a clarification/explanation or two (if it's lacking) will automatically bump it to the front page, possibly getting some more eyes on the post. That way, it doesn't look like anyone's soliciting votes, yet is a perfectly fine way of bringing something to attention.
We have very favorable numbers compared to most sites, but there's always room for improvement. That way, at least when you see a disparate voting, you can be fairly sure that it just happened to be posted at the wrong time and not necessarily due to apathy. A better solution for this would be to take a zen-like view on this whole "reputation" and "votes" thing and realize that the numbers don't really matter and you're here for the fun and to help. If I leave this place, it'll sooner be due to a toxic/unfriendly environment rather than a paucity of votes.
Perhaps we could have some place to feature questions that the community thinks need more attention and that have less than e.g. 5 votes.
It already exists — it's called the bounty system. Use it.