why is the shaft of femur bone convex frontly?

Answer 1

It's because of the way our knees bend. If they bent in the opposite direction, like birds, then our femurs would be concave. Think about getting into a squatting position, the force of your weight is more effectively transferred through your legs because of the anterior convexity of the femur.

Answer 2

If your femur was concave it would be really weird, funny, and perhaps very useful for putting things in, like salt when you eat chips, or pencils when you wrote a letter. Mmmmmm.