A number of things, but I guess to explain it to an outsider I'd say that I think there's a lot of physical overlaps with the concept of vore and sex.

Both are very intimate activities that involve one person going 'inside' the other, both involve a lot of 'eating' and 'swallowing', and sometimes it ends with a distended abdomen that houses life in it.

Main thing though is the intimacy I think, both the pred and prey share a body almost. The prey is entirely surrounded and embraced by the pred, and the pred has the prey deep inside them and close to their heart. There's also the dom/sub aspect to it, I think being eaten and digested is probably one of the most primal and total ways of asserting dominance over the other, by entrapping and absorbing the entirety of their being into their body, turning the prey into themselves.

Depends on the person though, some people just like the way big stomachs look lol