They could've chosen another Pokemon like Geodude, Ekans, Meowth, any of the others! Why did they choose Pikachu?
Originally it was Clefairy. They chose Pikachu because he was yellow and easy to spot. It was also unique as there were not many yellow coloured Mascots when Pokemon first became created
Initially both Pikachu and the Pokémon Clefairy were chosen to be lead characters for the franchise merchandising, with the latter as the primary mascot to make the early comic book series more "engaging". However with the production of the animated series, Pikachu was chosen as the primary mascot, in an attempt to appeal to female viewers and their mothers, and under the belief that the creature presented the image of a recognizable intimate pet for children. Its color was also a deciding factor, as yellow is a primary color and easier for children to recognize from a distance, and with consideration to the fact the only other competing yellow mascot at the time was Winnie-the-Pooh
