The courts are responsible for determining whether or not a law is constitutional. Courts look at legal precedents to determine this as well as their own reading of the Constitution. Justices determine if the law violates the spirit of the Constitution which is supposed to give everyone equal protection under the law. The justices can determine the laws as they deem best because they are not accountable to any political group. Since they do not have to run for re-election, they can determine constitutionality as they see fit.

Legislators, on the other hand, are responsible to their voter base. Someone with progressive racial views may have difficulty getting civil rights legislation passed if his/her constituents do not want it. Legislators also use compromise to get bills passed. If someone does not receive a benefit for their district, a legislator may find another bill to champion even though civil rights legislation would be the best bill to pass. The legislatures are larger debating bodies than the court systems so it takes longer for them to decide anything in general.

Congress had difficulty passing civil rights legislation during the 1950s and 1960s because of Southern Democrats who wished to preserve the racial status quo for their voter base. John F. Kennedy, a man with progressive racial views, initially did not want to upset his political party by pushing this legislation through, so much of it died in committee. The Supreme Court, on the other hand, issued rulings determining that poll taxes were unconstitutional and that segregation violated the spirit of the Fourteenth Amendment by denying people equal access to things that should be guaranteed to them by their citizenship.

There are several reasons why many of the advances in civil rights have come through the judicial rather than legislative branch, although the Civil Rights Act of 1964 is a major exception.

Perhaps the leading reason for this on the federal level has to do with something built into the Constitution, namely that smaller states at the time the United States was formed wanted to ensure that they would have equal representation with larger states. Thus in both the Electoral College and the Senate, voters from sparsely populated states have significantly more votes, as it were, than those from larger states. The 579,315 residents of Wyoming have the same number of senators as the 39 million people of California. As lower population rural states tend to be more conservative, this means that their disproportionate representation in Congress leads to votes that are more conservative and more opposed to civil rights than actually would be the case in a direct democracy.

In answering this question, one should bear in mind two important factors. The first is that, for many years, state legislatures, especially those in the South, were implacably opposed to civil rights. White supremacy was the order of the day, and so if any legislation were to be passed affecting the lives of African-Americans, then it would be to keep them firmly in a state of subjection, not to advance their civil rights.

The second factor one needs to consider is the domination of the Southern states in the United States Congress, which meant that those hostile to civil rights legislation had an effective veto. Republican presidents were reluctant to challenge the Southern bloc as they needed to secure its cooperation in getting their legislative agendas passed. And Democratic presidents couldn't make much progress in civil rights, either, even if they wanted to, because they relied on the then solidly Democratic South for reelection.

Legislators are concerned with being elected to office. Logically, this means that to be elected and remain in office, legislators campaign on and write bills on issues intended to please the majority of their constituents. Besides, Congress requires a majority of votes to pass legislation, and senators' filibusters can stop a bill from passing. The founding fathers intended for the legislative process to be slow, and these factors make it so.

Legal precedent is the practice of deciding a case based on decisions made in earlier cases. People view the judicial system as a faster and less arbitrary arena for determining the law, based on legal precedents, statutes, and the Constitution. It is the court's job to decide cases based on the established rule of law or determine a new outcome that will serve as ruling precedent in other cases. Decisions made in superior courts are binding on lower courts. This aspect is what makes their decisions less arbitrary and more reliable, especially if you've studied those precedents and feel that your case has merit. Some would argue that there is less politics involved in the court system, as some judges are appointed "for life."

Legislatures function through compromise.  Even when one party has enough votes to get a majority and so pass a bill, in the US Senate 60 votes are needed to stop a filibuster.  In addition to the mere votes, legislatures also depend on what is called comity.  Members are reluctant to trample on the deeply held views of a significant group of other Members, since they will no doubt need their support on something important to them in the future.  Finally, Members of Congress have to get elected.  Many of the advances in civil rights were needed precisely because blacks had no political power.  As a result of all this, radical change is often difficult to achieve through legislative action.

The US Courts, on the other hand, are tasked with interpreting the US Constitution.  Sometimes the Court is very conservative, but at other times the Justices feel that the Constitution demands a departure from precedent and tradition.  The fact that they are appointed for life makes this sort of controversial change far easier for a Court.