Largely, this is because sociology has tried to emulate (and has been influenced by) harder and medical sciences. Psychology and medicine have been overwhelmingly prone to studying disease instead of studying well organisms.
Sociology has done likewise for legitimization reasons – this is what others do and they are legitimate, and so therefore we are as well. However, it also does so for the same reason that medicine looks at disease – It is difficult to define what constitutes health.
What constitutes freedom? Marxists, for an easy example, can – and do – argue that anyone who essentially doesn’t agree with them is duped and uninformed of their true needs and desires. Your desires are shaped and prodded by group characteristics; you’re not truly free.
Further, individual sociologists (much like psychologists) tend to study that which fascinates them. The things that fascinate them tend to have echoes in their own lives. That is, one who felt held back at some point is likely to study that same thing.
Individual humans – at least, smart ones – are also more likely to presume that other people are as smart as themselves. Therefore, if others have not succeeded, it must be due to repression from other quarters.
Also, the solutions to restrictions are frequently much more palatable than the requirements of freedom. It is easier to find reasons why one was unable to study than to live up to the demands of studying.
Finally, we have been limited by the consciousness of our times. This is where sociology’s youth is its primary disadvantage; because the study is so young, its focus has not had time enough to change. Contrast this, for example, with the varying trends in philosophy – Spinoza to Kant to Hegel to [INSERT MODERN PHILOSOPHER OF YOUR CHOICE HERE], with very radically different worldviews. These worldviews are, in part, informed by the times they live in. While there has been rapid technological change in the last hundred years, our society and zeitgeist is just now starting to realize the changes that have occurred. Just as medicine and psychology are starting to study wellness instead of saying that wellness is the absence of disease, so I suspect sociology will begin to examine freedom as more than the absence of repression and restriction.