Because Gold and Silver predate the concept of the generation as an iterative process that replaces every route 1 shitmon and other staple Pokemon with alternative versions.
Johto Pokemon are almost unanimously tied to stuff like limited evolution items, headbutt trees, specific times of day, migrations, 1% appearance rates, off the beaten path locations, and Kanto because being able to succesfully train Johtomon was meant to be the mark of an above average player who had an adept understanding of the game. It was meant to be impressive if someone at your school had raised a Scizor, Kingdra, Houndoom, etc
