Why doesn’t the earth always show the same side to the sun?

Answer

The tidal effect does indeed slow down the rotation of objects. If the Earth did get the Moon in line and vice versa it didn’t, it’s simply because the Earth has 80 times more mass and thus causes 80 times stronger tides on the Moon than vice versa. I have already explained how this delay occurs (http://www.ikhebeenvragen.be/scientist/question3.jsp?id=6972).

We’ve got the Moon in line, but the solar system’s lifetime will be too short to synchronize the Earth’s rotation with that of the Moon.

With that of the Moon, indeed, and you are talking about the Sun. After all, the tides exerted by the Sun on the Earth are two times smaller than those exerted by the Moon. Weird perhaps, but it’s because the tidal forces decrease with the cube, not the square of the distance, and the Moon is so much closer.

If the Earth were much closer to the Sun, the slowing down of its rotation by the solar tides would be greater. Therefore, we think that many of those exoplanets that are very close to their parent star rotate synchronously with their orbits. Already in our own solar system we see that the inner planets rotate more slowly than those far from the Sun. But, as said, we are too far away for the Sun to get us completely in line.