From an ACT English test:
Now, the site is visited by thousands of scuba divers every year, those ____ for an unusual dive in what might be called an underwater mining museum.
Why inserting "looking" to make it a nominative absolute is more acceptable than inserting "who look"?
I think a very important aspect of this sentence is ignored in the other answer. As you have noted the sentence in question has an absolute clause in it.
If the sentence is
Now, the site is visited by thousands of scuba divers every year, those who look for an unusual dive in what might be called an underwater mining museum.
It still makes sense. "Those who look" is understood to refer to a general situation (there are always people who are on the lookout for an unusual dive) or a constant/habitual action (people who are always looking for an unusual dive).
Now, the site is visited by thousands of scuba divers every year, those looking for an unusual dive in what might be called an underwater mining museum.
With a present participle, the sentence becomes an absolute construction, "looking" being a nonfinite verb. (See how this sentence parallels your sentence?) In this case, "those looking" can mean either "those who look" or "those who are looking". That is just how participial phrases work. It doesn't have to only mean "those who are looking" but can also give a general situation.
So, to answer your question: why does the original sentence use an absolute construction as opposed to a relative clause. The simple answer is it is more versatile and more succinct. The sentence appears to refer to a general situation and can absolutely be reworded with a relative clause, but the absolute construction reads better.
"Those looking" describes people who, are at that moment, looking for something specific.
"Those who look" suggests that they are always looking for that something.
For example, if someone said, "I am looking for a reliable employee" that would mean they were looking right now for a single reliable person to employ. On the other hand, if they said: "I look for reliability in an employee" that would mean reliability is a quality they always look for in *all employees.
Another example would be the difference between "those looking for an intelligent girlfriend" and "those who look for intelligence in a girlfriend". The former means they are seeking one person; the latter suggests they repeatedly look for the same quality in a string of relationships.
As you can see, either work but with slightly different inferences. Your example sentence is about people paying a single, countable visit to a site it makes more sense to say that they were in that moment "looking" for what that site offered rather than assume that is what they always look for. For example, some people may look for a relaxing vacation one year, and then an action-packed vacation the next. You wouldn't say this kind of person is someone "who looks" for a specific kind of vacation. But if someone always chose action holidays, then you could.
Also, if the text is designed to generate interest and encourage people to visit the site then it must appeal to as many people as possible. If said that it was only for people who always look for what they have to offer that might be off-putting to those with a casual interest in it.
