It's an entirely different codebase. Lemmy is written in Rust, PieFed in Python. If you're a Python developer you're not going to randomly learn Rust to contribute to the huge existing codebase that is Lemmy, and even if you did you might have a different stance than the Lemmy developers calling for a fork.

Furthermore, it's not a zero sum game. The projects share content, and both benefit from the success of the other.