That is a good question because ash had the chance to catch articuno zapdos and moltres but he didn't and nobody knows why and he had the chance to catch others as well and I don't know why. The only thing that I could think of why he didn't is because they want to make the show last and make us wait for better episodes. That is the only thing I could think of.
Wat makes no cents is dat ud fink ashs pikachu would be unbeleviably strong (afta a few seasons). But wen pikachu versus Vigoroth 4 da 1st tym ash loses!!!! How is dat possible. Is pikachu just naturally weak???