>>47319646A mix of excellent marketing decisions(two versions, play along with Mew, anime synergy, we give Ishihara a lot of shit but he knows what he's doing, 99% of the times at least), being the actual starter(inb4 >muh DQ/SMT!) and the fact that it got so big in the West that everything that had any sort of remote similarity(to ridiculous degress like yugioh and medabots) were seen as clones.
And I'm not taking the question literally as you're probably thinking about Digimon as "monster collecting" which they're not because then the answer would be "its pretty much the only one around".
