The best answer for this is that Charizard is, along with Pikachu and maybe Mewtwo, the most recognizable Pokémon. It has been a fan favorite since 1998 and is still going strong. Blastoise and Venusaur are both iconic, sure, but not so much as Charizard.
Also remember, Mega Evolution was introduced in XY. X and Y were supposed to appeal to older Pokémon fans through nostalgia, bringing back a lot of appealing stuff from Gen I, the most obvious being the Kanto starters and Mewtwo. In order to strengthen the nostalgia, not only did they give the Kanto starters Mega Evolutions, but they gave the beloved Charizard two (they also did this with the equally beloved Mewtwo).
TL;DR Charizard got two Mega Evolutions because Pokémon fans, particularly the older ones, really really really love Charizard.
(Footnote: Venusaur is the best starter, in all seriousness.)