Effect of BMX bike geometry on manualing
As far as I'm aware, it's all about the rider's location vs. that of the rear wheel.

A short chain stay helps, because it tucks the rear wheel up closer under your center of gravity.

What really makes a big difference is the tilt of the handlebars: When the handlebars are rocked backwards, it lets you get your center of gravity way back past the rear wheel, making it really easy to lift the front wheel. In the same manner, tilting the bars forwards makes it hard to manual.