It seems to be intnetional. There are a handful of prestige classes out there (in Scrolls of Skelos as well as Shadizar), but for the most part the authors (in particular Darlage) have really taken advantage of the more relaxed multiclass rules to create different types of characters.
There are a couple of them in Aquilonia as well. I for one don't miss them at all. They can become obsessions for some players, building the PC just "this way" so that they can attain the class at the lowest level. The fluid nature of Howard's stories sort of dictates how the PCs progress in my game, not planning for Prestige Classes
Yes there is prety much an informal decree against PrC's in Conan. This has been mostly a reaction to the tendancy of most other game designers in the d20 market to use "prestige class" as code for "mechanic that makes the PC stand out and/or transcend the limits of the base classes". Conan is a swords 'n sorcery game; transcendence is achieved by becomming a legend that will live on in genetic memory even after the next cataclysm, no by sprouting wings from your back. And as for standing out; the base classes in Conan very much define the world and the people in it, the PC's are supposed to become a part of the Hyborian age, not an anachronistic import.
What few PrC's have been released have taken pains to follow the "join an orginization and benefit from its special trainning" design theory. I have few objections to that so long as the orginizations themselves fit the flavor of the Hyborian age.
In general I support Conan's prestige class detante.
However, I have a player who portrays a Gunderman and after looking at the Gunderland Pikeman Prestige Class it nailed his idea of the character he wanted to develop, rules wise. As the PrC is not unbalanced at all, I think both him and the game profited from having this precise PrC.
I also think the Professional PrC is, in some respects, a good option for making more "realistic" thieves; in fact, a Burglar is better done with this PrC as it doesn't get an outrageous sneak attack (more suited to an assasin IMHO).
Of course, PrC characters should be exceptions and I think the classbooks with the multiclass options are the way to go, as Hyboria's Fiercest and Finest have shown.
The way PrCs are handled in CONAN is IMO really good: created, where it makes sense, but not as fillery or to make über-characters. I hope Mongoose continues in this way!
The class mix concept by Vincent Darlage fulfills the same function, but is much more versatile and gives more ideas to players and GM thant the specialized D&D PrCs.
So happy there are no PrCs i think that Wotc were marketing geniuses to come up with it though. keeps there books healthy and thick adds to the price tag. i think they detract from the game as cooperative story telling. adds way to many abilities and drives the character to look at their sheets more than developing their character out of the own imaginations.