It's because your selector isn't specific enough. Try:
.row .large-12 .orbit-caption:hover {
background-color: red;
}
I'd suggest looking into CSS specifity (mdn).
You were using a selector with a specificity value of 20
whereas the selector you were trying to overwrite: .orbit-container .orbit-slides-container>* .orbit-caption
had a specificity of ~ 30
.
The selector .row .large-12 .orbit-caption:hover
has a speciity of 40
(note the pseudo class)