Steveyboy,
Please don't make me Google for it; but apparantly step ladders are 'our' most dangerous ladder. I'll Google if challenged about it! Watch yourself, mate!
Honest I will.
Also, steps are bulky to carry around with your double extentions. I'm not sure how you carry them about with your bucket, but we use a large unger bucket and once that's full of water, a couple of ladder mats, sometimes a Rojack ladder stopper and our sprays - it weighs a ton. No chance for a step ladder to fit in or around it.
All you have to do with an A Frame is to place it against your 'big' ladders and carry both in the same hand! They're far easier to use - no opening or closing - look more professional too.
Get the 'big feet' (what're they called?) on the bottom of your A Frames and they cause almost no damage to 'planted areas' in front of living room windows. They're compact and well; I wouldn't be without mine.
But, Steveyboy, I think you answered your own question, when you asked 'Why do so many use A framed ladders?'.
Safer and easier!