Why don't observers on Earth ever see the back side of the moon?
Observers on Earth can't see the other side of the Moon because it is tidally locked.
The Moon only shows one face to the Earth because its rotation period is the same as its orbital period.
When the Moon was first formed it was much closer to the Earth and rotating about its axis much faster. The Earth's gravity slowed down its period of rotation. It also slowed the Earth's period of rotation. This also transferred angular momentum to the Moon's orbit causing it to move further away.
Ultimately most moons become tidally locked to their parent. This means that their day and their orbital period are the same. This means that a tidally locked moon always presents the same face to its parent.
In fact we can see about 59% of the moon's surface from Earth. This is due to a wobble called libration.
Curiously as the Moon is of significant size, it is also in the process of tidally locking the Earth. If this process were to complete, the earth and Moon would always present the same face to each other. This will never happen as the Sun will have become a red giant before this happens and may have consumed the Earth and moon.