Hi -- great question! It all has to do with geometry. In a nutshelll, when the Earth blocks the Sun and the Moon passes through the shadow (a Lunar Eclipse), the moon is close enough that there's a fairly large area for the moon to pass through and end up getting eclipsed by. Additionally, it is generally visible from a large part of the Earth. When the Moon blocks the Sun (Solar eclipse), there's only a tiny bit of shadow, so only a small path on the Earth gets to go through it.
I found a great image to illustrate, courtesy Rice University - hmm, I can't seem to cut & paste it in here, but see
http://space.rice.edu/ecl...nar_eclipse_faq.html
about a third of the way down.
-KD