Because the area on Earth that's inside the tip of the moon's shadow is
usually only a couple of hundred miles wide at most, and it's moving along
the Earth's surface at thousands of miles an hour, and only the people who
happen to be inside that little area see the total solar eclipse.
Over a substantial period of time, the numbers of solar and lunar eclipses
are equal. For example, during the 100 years of the 20th Century, there
were 128 solar eclipses, and 129 lunar ones.
The difference is that during a lunar eclipse, everybody on the night side of
Earth can see it, but during a solar one, you have to be in just the right place
on the day side.
Another way to look at it:
A lunar eclipse happens on the Moon, and anybody who can see the Moon
can see the eclipse. But a solar eclipse happens on the Earth, and in order
to see it, you have to be where the eclipse is.
