I believe low hanging fruit is not specific to CS.
Regardless, it is a perfect metaphor. You want to eat an apple: which one do you pick? Taking the low-hanging fruit is less work right now and gets you to your immediate goal, but disregards general efficiency. Sure, picking a whole tree is more efficient. But if you want a single apple, taking the low hanging fruit is the fastest approach. The metaphor works because it actually implies that it's not the most efficient approach, just the easiest
Everyone, even the amateurs and complete non–gardeners, know that this is not the best way to pick fruit. The whole point of the phrase is to point out that someone was lazy. It is saying that all they did was the absolute minimum amount of work.
Regardless, it is a perfect metaphor. You want to eat an apple: which one do you pick? Taking the low-hanging fruit is less work right now and gets you to your immediate goal, but disregards general efficiency. Sure, picking a whole tree is more efficient. But if you want a single apple, taking the low hanging fruit is the fastest approach. The metaphor works because it actually implies that it's not the most efficient approach, just the easiest