Mate it's more so because of habitat preference than anything. Yes, Red Bellies do include other snakes in their diet, but the biggest reason is because Red Bellies are predominantly a frog eater, so they tend to stick around water courses and swamps, whereas an Eastern Brown will eat whatever it can fit down it's mouth and is a lot hardier than a Black, so it occurs everywhere apart from dense rainforest.
I have a challenge for everyone...instead of freaking out next time you see a snake, just stand there and watch it. Not from a safety point of view, but just check out how amazing they are. They really are a beautiful family of animals. If you want, swing me a few photos and I'll tell you what species of snake it is - we have over 130 in Australia!