1. Cum eating is considered riskier for HIV transmission. Straight productions require regular testing, gay companies do not, so they are limited by no-fluid exchanges only.
2. Straight porn has more emphasis on male dominance over women. It's considered degrading to cum in a girl's face or make her eat cum, and guys love treating girls like whores.
3. Straight porn is, duh, about the girls not the guys and their cum. The straight viewer doesn't really want to see the guy cum, he wants to see the GIRL reacting to it. (Watching a guy cum? That's gay!)
4. They frame the cum shot so the girl is the focus, and only show the least amount of the guy's body. If the shot consists of just a disembodied dick and a girl's face, the director says, "Make her eat the cum."