Pokemon has always been an average JRPG series. However, there are two main reasons why it was so successful. The first is that collecting and training lots of different Pokemon is a nice concept. The second is that the original games had a fresh multiplayer experience. Most multiplayer games before Pokemon involved either taking turns or going head-to-head. Pokemon allowed you to trade with your friends as well as battle them, which encouraged people to actually catch 'em all.
And since the original games were so popular, their sequels managed to be popular too. The concept is also very translatable, so they could easily make related TV series, movies, video games across multiple genres, merchandise, cereals, etc.
