Because the moon's orbit is slightly inclined with respect to the earth's orbit around the sun. There are two places where the planes of the earth and the moon's orbits intersect these are called the nodes. It is only when full moon occurs when the moon is close to one of these nodes that an eclipse of the moon can occur. This is similarly true for new moon occurring and resulting in an eclipse of the sun. There are therefore two periods each year when eclipses of the moon and the sun are likely. The nodes of the moon's orbit gradually move with time completing a full circuit about every 19 years so eclipses gradually move through the year and tend to repeat in a regular pattern every 19 years. The ancient Greeks knew about this and could predict eclipses reasonably accurately. I also have predicted eclipses back in the 1950s based on information given in the Nautical Almanac and a book Called Practical Astronomy by W Schroeder.

There are typically 4 eclipses a year two of the moon and two of the sun and in 2008 they will happen in February and August.