Why does the supreme court sometimes reverse its earlier decisions?

Answer 1

Societies change over time. Practices considered acceptable - even righteous - might be seen in an entirely different light two generations later. These practices generally involve moral principles. So, the Supreme Court once in a while weighs cases against the Constitution without taking the larger moral question into consideration, especially when society at large has settled on one side or the other.

In the past, the Court has ruled cases involving child labor, racial inequality (referring here not to cases of people being treated differently because of race, but to the notion that non-white races are themselves unequal and inferior), certain romantic activities between adults, and slavery could not be condoned or overturned on constitutional grounds.

Sometimes the issue isn't moral, but regulatory - the Court has sided against a New York State law limiting a baker's work week to 60 hours and overturned a law setting minimum wages for women. Sometimes the issue is political. The Court has reversed itself regarding decisions on campaign finance and voting rights, for example.

The Court still supports decisions that it could reverse in the future. Although many today would deny it until their final breath, it is hypothetically possible that society might one day decide to prohibit abortions in all but the most extreme circumstances. A future Court examining the ruling of the Affordable Care Act might decide it had, in effect, re-written the law in order to deem it constitutional. That in itself is unconstitutional because Congress writes legislation, and never the Supreme Court.

Answer 2

When new justices are appointed, their interpretation of the law may be different.