I read somewhere that these type of attacks are susceptible to addresses that are higher and not lower in the stack/heap/buffer. Why is this?
It's because of how data is stored in memory.
When a data object larger than a single byte needs to be referred to, programs do so by remembering the address of the first byte, plus (in some fashion) the size of the object. That first byte is (by convention) placed in the lowest-valued address of the available space, and as a result, any overflow will go off the high-addressed end.