На форуме иностранцев, изучающих русский язык, задают распространенный вопрос:
[Spoiler (click to open)]
И, кстати, ответ на него будет полезен тем, кто учит английский:
This is somewhat close to question: "Why does in English "awesome" means great but "awful" means something horrible?". The short answer is: the fact that a pair of words is derived from common root does not mean that these words do not have opposite meaning. Moreover, the same word in different context can stand for completely different concepts.
For example, in English word "terrible" in colloquial speech can be interpreted as "formidably great", compare to "ужасно круто" / "ты ужасно красива". Recall English usage of "fucking", it is used to indicate an extreme degree of something either positive, or negative.
As for the question why exactly some forms acquire positive connotation ("охуенный", "пиздатый") and some negative ("хуёвый", "пиздецовый"), we can only explore when exactly it becomes common, but not why, I guess. As far as you are asking about why but not about when, I do consider this question answered.
To put it simple, it's not something specific to Russian and just because :)