For me it's the angle of the dangle.

I'm most comfortable standing to pedal on my hybrid with riser bars at saddle height. I can stand upright, straight over the pedals. It transfers my weight efficiently to the pedals. The downside is it's less aero so beyond a certain speed, or climbing into headwinds, it's not faster. It's just comfortable for longer distances.

Until recently my road bike had a long stem. I was too stretched out and when I stood to pedal I was leaning too far forward. It was more aero, but I had to physically push back to pedal. My weight wasn't transferred as efficiently. I'd become exhausted more quickly.

Recently I swapped the road bike to a shorter stem. Much more comfortable sitting and standing to pedal. My weight is better balanced when standing to pedal. I can pedal more efficiently, longer and without much compromise in aero positioning. For me it's more efficient for short, steep climbs. And it feels like I can transition between sitting, standing, sitting again, along with using the downtube shifters, without losing momentum. That's primarily a balance thing.

FWIW, my road bike is an old school '80s thing, 7-speed freewheel. Currently 13-24 original, and I have a 13-25. I swapped the 52/42 chain ring for a 39T small chain ring. It's still an old school combo, not really suited to serious climbing. But we don't have any serious climbs around here, nothing longer than a mile of continuous climbing at worst. Most of my climbs are maybe 100-400 yards of continuous climbing with plateaus or dips before the climb resumes. Lots of roller coasters with those 4%-9% or so short steep bits. For a strong rider that 52/39 chain ring with 13-24 freewheel is good enough. But I'm not a particularly strong climber. I'm tempted to try a 13-28 freewheel, but I know the spacing would be weird, too jumpy and herky jerky. So either I get stronger with the gearing I have now, or switch to a newer bike with more gears and better spacing.