Part of the reason would be that the new PlayStation 4 allegedly uses AMD x86 chips, so direct backwards compatibility would not work, forcing Sony to use software backwards compatibility that Microsoft used with the Xbox 360.
The WSJ also stressed that the live streaming would only apply to PS3 titles and not new PS4 titles.
Several questions arise, however. Would gamers be forced to repurchase PS3 titles, much as they had to repurchase PlayStation 2 games after the Emotion chip was removed from post-launch PS3's?
Then there's the structure and philosophy of Sony itself as an electronics company. After the acquisition of Gaikai, an analyst:
"Sony buys it, because cloud gaming is an existential threat to a core part of their business. They then try and integrate it, but they integrate it Sony style - they integrate it and all of a sudden they have to work with the Sony Online Networks, and it has to be integrated into Bravia, and it will only work on certain televisions and not others, and it will have to carry PlayStation branding and you won't be able to use it if you're not building a PlayStation game."