Why are two seismic stations not enough?

Because you need three points to locate the source of a signal.

Think of a station as the center point of a circle. The station can detect the seismic wave, and the wave's properties can tell you the distance (P and S wave measurement) but not the direction it came from.

One station would give you a whole circle edge (circumference) to search for the wave's source, based on that distance (radius of the circle). But if you have a second station, you can measure the same wave there and get a second circle radius from that station. If you draw both of these circles on a map, with the stations at the circle centers, you will see that the two circles intersect at two points. This narrows the possible wave source to two places.

But if you have three stations, and draw a circle around that station based on the wave measurement, there will only be one point where all three circles intersect. That point is the source of the wave the stations detected.