>>44276773Probably through coincidence. A GF employee have said in an interview for gen V that the team was commissioned to create a powerful three-stage dragon line, and that this was expected. They likely created the Beldum line late into development; it has a late index number and was given as a postgame gift pokemon instead of being found anywhere in the wild.
The most logical explanation was that they wanted a powerful ace for Steven so the supposedly 'steel' champion had more than 2 pokemon of that type, and it needed to be powerful so they said fuck it lets make another pseudo. The johto games that came before it had both dragonite and tyranitar obtainable, and neither one of those two appeared in rse, so it makes sense they'd be fine with having 2 in those games as well.