Why were freedmen not able to enjoy the freedom that they received as a result of the 13 amendment

 
The 13th Amendment to the United States Constitution states as follows:

Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Section 2. Congress shall have power to enforce this article by appropriate legislation.

By "freedmen," the generally understood meaning is former slaves who were under involuntary servitude (slavery) to slave masters in southern states where slavery was legal prior to the ratification of the 13th Amendment. Perhaps the most significant reason why many freedmen were unable to enjoy fully the freedoms provided under the 13th Amendment was the substantial efforts many (if not all) Democrat-controlled states in the South (the Confederate States) immediately undertook in a deep-seated effort to preserve a culture of racism.
As a direct response to the passage and ratification of the 13th Amendment across southern states, state and local jurisdictions in southern states enacted a series of racially-restrictive statutes, ordinances, and rulings (commonly referred to as "Black Codes"). For example, without the ability for now-former slave masters to lawfully secure labor at low or no cost through the mechanism of slavery, the assumption was that Black Codes permitted a way around the intent of the 13th Amendment while in principle not directly violating the 13th Amendment. Black Codes went far beyond merely creating barriers to reasonable wages and fair labor laws but also affected the ability of blacks to obtain and enjoy such rights as equal access to housing, a fair and speedy trial, the right to bear arms, the ability to attend public schools, and a host of other rights provided to non-black citizens of the United States.
State and local laws viewed as Black Codes, in many cases, were painstakingly-crafted not to be outright racist against blacks on their face. However, the application of such laws created a net effect that such laws were in fact racist in a way that unfairly, unjustly, and unconstitutionally violated the rights of blacks to access and benefit from equal rights and liberties as enumerated under the United States Constitution.
That said, many of the state and local laws passed following the ratification of the 13th Amendment outright prohibited blacks from enjoying equal rights. Many of these Black Codes were quickly challenged at the state and federal level as unconstitutional and subsequently repealed. However, many of these laws remained in full force and effect for a number of years to follow. Southern states also worked around the invalidation of Black Codes by coming up with a more insidious plan to preserve de jure segregation in most sectors of society. State and local laws (commonly referred to as "Jim Crow laws") enforcing racial segregation for the remainder of the 19th century until 1965 continued to restrict the civil rights and civil liberties of blacks, particularly in southern states where the bulk of Jim Crow laws existed.