The short version is that Pazio did a lot to encourage class loyalty. Consider the 3.5 Sorcerer. You have one class ability, at first level. After that, your only class progression is in casting. Well... there are PrCs that can progress casting /and/ give class abilities. I'm inherently better off going to one of them as soon as I can, aren't I? I can keep getting the same stuff (casting) AND get extra stuff on top (actual class features).
Compare the Pathfinder Sorcerer. You have a single dead level at level 2 (which is extremely rare in Pathfinder to begin with, for the record), but every other level you're either getting a new level of spells or a class ability (pretty much all bloodline based). And some of those bloodline abilities are very, very good-- there's a lot of salivating over the Arcane bloodline, in my experience, though Sylvan is also awesome and a few more (Draconic, Orc, Primal) see good use because they add raw damage to blasts.
So to seriously consider prestiging out of Sorcerer, I either need a PrC that can progress my casting AND my bloodline, or I need features that are inherently better than my bloodline. The former has one, the Dragon Disciple. It only progresses your bloodline if you're Draconic though, and costs a caster level at 1, 5, and 9. Not a /bad/ choice... but most certainly not an auto-grab, even if you're among the small subset of Sorcerers who can consider it in the first place. The latter doesn't really have anything, because PrCs are generally not as good as they were in 3.X. Finding a full-casting, full-BAB prestige class in 3.X took me roughly thirty seconds... while one doesn't exist at all in Pathfinder.
