Because if SCOTUS gives up power in one area of Constitutional law, it opens the door for them losing ruling power over the whole Constitution.
This is bizarre. Your statement presupposes that SCOTUS already has “ruling power over the whole constitution” (whatever that means???) AND ignores the entire concept of judicial review. Ever since https://en.wikipedia.org/wiki/Marbury_v._Madison in 1803, SCOTUS has had the job of interpreting constitutional law for both the federal government and individual states.
Also, SCOTUS is not, as we have recently seen, bound by stare decisis at all. Who or what power is going to bind SCOTUS to previous rulings? SCOTUS has overruled its own prior decisions time and time again, and lost absolutely nothing.
Furthermore, in the United States, the only power constitutionally able to check the judicial is the legislative, and Congress can’t just overturn SCOTUS rulings or demand that SCOTUS not hear cases brought before it.
SCOTUS derives its powers directly from the US Constitution and has already proven over the last 200+ years that it does not gain or lose any powers whatsoever by interpreting (and reinterpreting) that law as requested.
EDITED TO ADD link to Marbury, above, which Wikipedia describes as “the single most important decision in American constitutional law.”