One reason is that the dualshock digital-pad is one of the most horrible, painful, unusable, piece-of-crap digital pads every made. Sony has always been crap at controller design, but the digital pad is bad even for them.
Have you actually used a Dualshock 3 or are just going by past experience with Playstation controllers?
I've actually used a ds3 (I own one). I grant that it may suck less than previous playstation controllers (the PS1 controller, for instance, was so mind-numbingly horrible that they really couldn't go any direction but up...), but it most definitely is still an awful dpad.
The Dualshock 3 is a really solid controller. Sony must have learned.
It's ok (nobody actually uses the dpad anymore after all), but "really solid" is being far too kind -- and given Sony's long history of completely fucked up controllers (helloooo psp...), it's hard to think the reason is anything other than dumb luck.
[Even if the shock of being solidly in last place has made them actually start to care about what their users want (something which they clearly never did before), it's very hard for them to suddenly pick up and start excelling in an area they previously ignored -- it will take time and experience to do that. Whether Sony actually has much time left is a very good question of course....]