I would summarize the HKG SKL's popularity to just one word, location. It's more convenient for most *A pax because it's located in the main hall right after exiting immigration. I prefer the UC much better; however, it's a train ride away. That's a deterrent for many because the train only runs one way. If you have a flight assigned to a low-numbered gate, you'd have to cut your lounge time short and prepare to hike.

In a similar vein, CX's Wing F/J lounges, which are also located in the main hall on the opposite side of SQ SKL, are much more popular than the CX Pier F/J. But that says nothing about whether the Wing is "better" than the Pier. Personally, I'll take the Pier experience with a 20-minute hike (not exaggerating) back to the main hall over the Wing any time.