Why is it that only Prof. Oak became a major character in the series, so much so that we see him in every region with Ash making phone calls? Obviously Ash leaves his Pokemon with him and he's related to Gary, (although we barely see Gary), but why did the writers make such a big deal out of Oak?
With Prof. Elm, Birch and Rowan, we see them make a few appearances throughout their regions but they're not major characters at all. After Johto ended we never saw Elm in a speaking role again, and likewise since the end of Hoenn we never saw Prof. Birch again. Its also likely after Sinnoh we'll never see Rowan again.
So what made Prof. Oak so special that he continues to appear in every series, even in the videogames he makes a cameo in DP, and the other Professors are treated like crap?
With Prof. Elm, Birch and Rowan, we see them make a few appearances throughout their regions but they're not major characters at all. After Johto ended we never saw Elm in a speaking role again, and likewise since the end of Hoenn we never saw Prof. Birch again. Its also likely after Sinnoh we'll never see Rowan again.
So what made Prof. Oak so special that he continues to appear in every series, even in the videogames he makes a cameo in DP, and the other Professors are treated like crap?