There is a National data set called the NPSAS that has a great deal 
of data on post secondary education and also has GED as a 
variable.  Here is some data from the NPSAS that partially explains 
why GED students are less likely to complete.

When it comes to postsecondary education then, adult literacy 
education graduates struggle.  They are less likely to enroll than 
traditional high school graduates.  They are less likely to complete, 
and in community colleges, they are more likely to be assigned to 
developmental skills The questions is: why?

A large part of the answer comes from the simple fact that they are 
adults and have all the responsibilities of adulthood that interfere 
with education.  While the mean age for traditional high school 
graduates enrolled in postsecondary education is 26, the mean age for 
GED graduates is 32.  GED graduates begin postsecondary education 
later than traditional graduates. While the mean age that traditional 
graduates begin postsecondary education is 21, the mean age for GED 
graduates is 27 (NPSAS 2004).

Thirty-two percent of GED graduates enrolled in postsecondary 
education are married compared with 20 percent for traditional high 
school graduates. Forty nine percent of GED graduates have dependent 
children; twenty-three percent of traditional high school graduates 
have dependant children;   While on the average traditional high 
school graduates receive $857 in financial aide, the figure for GED 
graduates is only $165 (NPSAS, 2004). If you are married, have 
children and have to make ends meet, of course postsecondary 
education is a struggle.