Gen 5 still sold over 15 million copies. It may not be the most popular generation, but it was by no means a failure. The reason they focus heavily on Gen 1 is because of branding/marketing. The roster has gotten way too bloated and they don't expect new fans to know every single Pokemon. But they want them to be familiar with the first 151 because everyone knows them. It doesn't matter if you started in 96 or 2023, you know who Charizard, Gengar, Eevee etc. are.