In that sense I always found it weird that promotion for the movie focused so much on Flik/Heimlich.
Heimlich was the easiest to get a gag across about in 2 seconds for a commercial. "He's a fat caterpillar with a funny voice!"
They also ran the "lady" bug joke to the ground in the commercials because again, easy to explain gag. (That also happened to be his only personality trait)
Flik was advertised because… he was the main character.
@Foolio:
As a side-note I wonder if Arlo was meant to inbreed since as far as we know there's no other civilization within miles of the farm.
I'm going to say no.