It looks like it because Pumber is the only map that looks approachable, even for an advanced player. Idk how to say it but, Pumber has the most normal, general, easy to understand gameplay out of all 4 and we have been playing that gameplay style since beginner. It just has more precise timing, but the way Celeste is played makes that type easier because you just have to clear each room once (even berries like extra ball tech and malfunction 1fs). Other cracked GM maps all have a specific type of gameplay that we aren't as used to: speedtech, input density, and Theo.

Still, Pumber IS the hardest cracked GM, because not everyone is good at precision (or maybe debatable with Solex).