OP, tbh it's simple. Sony did a survey and realized that people rarely use BC vs playing newer games. It's a nice feature for sure, which Sony paid extra cash and invested in more silicon to deliver to PS gamers twice in a row on PS2 and PS3, but at the time when they had to revise their PS3 plans from the 60GB fat console, they did surveys as to case by case uses. Do people really play old games, did they really want an other OS or SACD. Sony is all about giving gamers the best, the most cutting edge tech and being the nice guy, but when they were on the ropes with PS3 with a $600 system which was the butt of many jokes from XBOX fans like "George Forman Grill", "The Amirox nervous breakdown", they had to dig deep and be more careful going forward.
They devised the slim which slimmed down on BC a bit and a few other features and realized that gamers just want a fast system at launch that can give them the best looking next gen games possible.......No TV TV bs, no high focus on extras. In essence make sure gamers can see some real next gen games and most importantly, that it's affordable to them.........It's why their PS4 was both cutting edge with 8GB of GDDR5 and a bonafied 1080p system with 32 ROPS in 2013, but moreso gamers got that at $399. Sony did it's homework as it did several surveys during the PS3 era, apart from the BC one, they did one on pricepoints and realized that price is very important to the consumer....All these lessons they were taught from PS3..........
I can tell you regardless, Sony is the good guy of the industry, because had PS4 been on an advanced Cell CPU, PS4 would have had full BC with PS3 regardless of the survey, but there's no way the jaguar can emulate Cell when very fast and expensive Intel processors have had trouble doing it....Yet we know Sony will deliver a feature if it's possible, that's why there will be PS4 BC on PS5 as a guarantee......
But yet the general question of why? .....And truth be told, I wont even use the Sony Survey stats to show.....I'll use the very company you believe is ace at BC, thinking that they are doing it and touting it's horn because there is huge demand and constant playthroughs of old games? There.......
They are only using it because Sony did not invest in BC this gen because of hardware conflicts, not because there's huge demand.........