I'll take a stab at it. I can see several reasons why many people go frameless. First, there are very few packs with stays in them that are really light, so the penalty for a frame is usually a couple of pounds. Second, most of the people who go frameless do so after having gotten their base weight fairly low - under 10 lbs - and in many cases are sloe doing relatively short trips - 4-5 days - so that their total load minus the pack is less than 20 lbs and often closer to 15. For a lot of people - though not everybody - 15 lbs is pretty comfy in any pack, so why pay the weight penalty to get a frame?
Plus I think not many people have tried a really wide range of packs. If all you have experience with is a 6-lb framed pack and a 12-oz frameless, then the anaylsis is different from comparing that same frameless with a 16-20 oz pack with stays in it.
For myself, since I make my own packs and have been doing so for 35+ years, I've tried all the variations from external frames to internal frames, to framesheets, to completely frameless, foam pads sewn in or stuffed in, day pack that zips ontop of a big fanny pack as hipbelt, you name it. The pack I use now has stays and weighs 16 oz, and I far prefer it to any frameless pack even at 10 lbs total weight. But I see nothing on the market that compares to it, so I can see how it might be a different decision if I had to buy a pack.
And I should say I'm at the low end of the spectrum in tolerance of weight on my shoulders, which makes my less tolerant of frameless packs since they don't transfer weight to the hips as well. Others, who are more tolerant of weight on the shoulders, see it differently.
Of course, there's one other reason - they are all the rage.