Digimon was cool but I think it suffered from a few things:

• Niche appeal
  Digimon really only appealed to those "nerdy" enough to delve into anime and accept lots of convoluted anime tropes. It was also constantly about being the chosen ones to save the world.
  Pokemon on the other hand was excessively simple and felt more like an american cartoon than an anime in a lot of ways. Pokemon's biggest success is that they save the crazy "chosen one" stuff for the movies.

• Designs
  Digimon designs were too... uncreative? I really don't want to say that but I think that's why people preferred Pokemon more. When your flagship mascot is a midget yellow t-rex, it's hard to appeal. It also doesn't help that the protagonist looks like a human version of Sonic the Hedgehog.
  Pikachu didn't look like any specific animal, (is that a mouse/rabbit/dog? no it's a cute pokemon) and that is what lends pokemon its design success. Also people could argue "what about Charmander just being a red lizard" well he still had unique traits like the fire tail, and didn't just evolve into a bigger charmander with a helmet (Agumon's evolution was that pretty much), he evolved into Charizard which is one of the best designs in all of pokemon.

• Theming
  Digimon didn't have types. You just power up or something? This makes it closer to Yugioh than Pokemon.
  In Pokemon you have Ghost types with a ghost theme, fire types with a fire theme, etc. This gives a clear and fun battle system with advantages and disadvantages.

• World Building
  Digimon is limited to a few people. They mostly exist in a digital world... which disconnects it from people other than the protagonists from having them.
  In Pokemon, they exist throughout the entire world, and everyone has them. Pokemon aren't just limited to Ash and crew. Could you imagine how boring that show would be? Digimon was able to roll with what they had, but Pokemon had way more potential easily because they didn't limit themselves like Digimon did. Pokemon are everywhere!