>>35760012While Ralts itself was a rare find on the second route of the game it was introduced, and it and Kirlia are fairly weak with Ralts having zero attacking moves when you first caught it in Gen 3, the prologue/tutorial portion of the game blatantly pointed out HEY, THERE'S A POKEMON YOU HAVEN'T SEEN HERE, and the final battle with Wally showed people what they'd get for sticking with the program- a powerhouse that rivaled Alakazam.
However, that wasn't what made it POPULAR. It- sorry, SHE- is popular because she's the first waifumon Game Freak ever put out. People sexualized her like crazy.