Here is a video that briefly explains the pikachu decision. That series (Did you know gaming) is very thorough and explains a lot of pokemon history as well as other notable games.
Now imagine Magikarp being Ash’s starter, twenty episodes on Magikarp doing absolutely nothing while Ash frantically tries to capture something else on his own without fighting. Then since Ash is one of the worst trainers ever considering he goes against the “gotta catch 'em all” motto releasing every other Pokemon he gets and avoiding capturing even fainted, legendary Pokemon he gets to meet, like Zapdos for example, Ash would probably release his captured Pokemon and then it would be season 2 of Magikarp’s splash attack.