But what about momentum? Lucio is definitely a prime candidate for an easy on fire character but it's not because he can push people off, it's because he stays on the point, heals everyone in range, and gets free kills just by tagging anyone who attacks the point while supporting his team. So once he gets on fire, he stays on fire. Even if he dies, unless the entire team is wiped, he's back into action quick and doing all of the above again, and stays on fire.
Pharah doesn't benefit the team by standing on the point or payload. She has to be out there in the sky flanking enemies or watching for their approach so she can strike from where they don't see her. All that is time being contributed to losing fire points while not gaining any until the action starts. She's actually one of the characters I consider hardest to stay on fire.
Same with Roadhog. He might easily get on fire by killing people, but he's also an easy target to get killed because he puts himself out there and is often the first to be killed if he's actually causing any damage. He's also slow, and for his hook to be effective he has to flank, which means going off point. Compare this with Reinhardt standing on the point blocking for his team. As long as he stays alive, he has to do very little that puts himself in harm's way to stay on fire.