Michael Shermer in SCIENTIFIC AMERICAN suggests that species just may
not be equipped to survive for long enough periods in large populations
to achieve the advanced technology needed to travel or communicate across the heavens.
He references the suppositional Drake formula N = R fp ne fl fi fc L for estimating the number of advanced civilizations just in our galaxy.
Shermer states that modern civilization lifetimes (L) on Earth so far approach only several hundred years. Applying that to the whole galaxy would yield a very small (N), or number of civilizations with the technical potential.
Source: SCIENTIFIC AMERICAN, August 2002