Why does @media min-width not override my CSS?

Try adding the !important flag to your CSS.

@media (min-width: 600px) {
    p{
        font-size: 1rem !important;
    }
}

Class selectors take priority over tag selectors (re: https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity, https://www.w3schools.com/css/css_specificity.asp).

Add a class to the p you'd like to change and target that.

.main__text {
  color: red;
  font-size: 2rem;
  margin: 1rem 0 2rem 0;
}

@media (min-width: 600px) {
  .main__text__alt {
    font-size: 1rem;
    color: blue;
  }
}

<p class="main__text">Some Text</p>
<p class="main__text main__text__alt">Some Text that will change color and size at 600px</p>

I'm not sure about your html but change the 'p' selector to '.main__text'.

try this:

.main__text {
  color: red;
  font-size: 2rem;
  margin: 1rem 0 2rem 0;
}

@media only screen and (min-width: 600px) {
  .main__text {
    font-size: 1rem;
  }
}

An easier way to understand media queries would be trying this:

.box {
  display: block;
  background: red;
  height: 100px;
  width: 100px;
}

@media only screen and (min-width: 600px) {
  .box {
    width: 200px;
  }
}

<section>
  <div class="box"></div>
</section>

Also keep in mind that 'min-width' and 'max-width' act oppositely... 'min-width' applies those styles when the screen size becomes larger than the specified unit while 'max-width' applies the style when the screen size smaller than the specified unit.

For the @media to work you need to specify something like screen like this:

@media only screen and (min-width: 600px) {
 p {
  font-size: 1rem;
 }
}