-Is a starter, they're typically popular as they're pushed and commonly used among players. -Is a (not) dragon, which of course does help. -The anime, Ash's Charizard is a very notable character from it. -One of the mascots for Gen 1, which is the most remembered of the Pokémon games. -Heavy pushing by GF, especially as of late.
There may be more reasons as well, but those seem to be the most common given for it.