Why are keycaps gated in the front and not the back where it isn't visible?

The side of a keycap opposite the gate is usually the last area for plastic to fill. The material flowing to this point comes from two directions as it travels around both sides of the key. When the flow masses meet, they create a vertical ‘weld’ line that is often cosmetically objectionable, especially on darker colors. Gating at the front of a key helps keep this imperfection off the viewing surface. The secondary problem is that not all the legends fill properly. Below are pictures that illustrate both non-fill and the weld line.