Simplicity, for one. with Pokemon, you had 3 evos, tops, and only a handful of branching evolutions. Digimon had long, differing branches of digivolutions
Marketing plays a role, too. With each gen , you get a set of Pokemon, and they beat those pokemon into your head for the most part. Movies around new legendaries, episodes around new Pokemon, toys and merch of new pokemon, making them prominent in games. You knew there were 151 Pokemon when it came out, but how many people knew how many Digimon there were? they were also much more unceremonious about new additions to the monster list, except for high tier digivolutions.
Finally, the design. digimon aims towards boys, so a lot of the designs are meant to appeal to boys. even cute digimon are designed so that they can look tough and mean.the digimon paired with female MCs look like they can appeal to boys, too; not as overtly feminine as jigglypuff or gardevoir, for example. Meanwhile, you have pokemon at all ends of the spectrum, and they're all featured to an extent. with digimon, the lesser or joke Digimon were glossed over a lot.
