In order for the courts at any level to hear a case, there must be an actual controversy between both parties. Since the 25th Amendment is all about Presidential Succession and at time of writing, no President has died in office since the law was passed, no one has had actual controversy such that standing to be heard was established.
Conversely the 14th amendment was basically patching loopholes in the Constitution that brought about the Civil War, which occurred due to a Constitutional Crisis brought about by Slavery and which level of government had the right to regulate it. The 2nd Amendment actually doesn't come up often before SCOTUS (it's had a few cases heard recently, most notable being Heller which was a landmark case because it was the first case to really get a 2nd amendment ruling (what's crazier is the 3rd Amendment is the only Amendment to never get a name check in a Supreme Court decision. Generally, the Amendments 1 and 4-8 get the most rulings because they have a lot of people who have standing to fight them, and 14th basically is a coverall that enforces states to recognize rights granted by the feds.
Basically the reason there is not much from SCOTUS on the 25th is because it hasn't really come up.