Aristotle was pointing down to indicate he believed things originated on Earth, Plato was pointing up to indicate that he believed our understanding was beyond our earthly understanding.

Aristotle was an idealist pointing beyond our capacity to understand without outer influence. 

Plato was a realist where we all live in, and get understanding from, the real world. 

Plato was pointing down to indicate he believed things originated on Earth, Aristotle was pointing up to indicate that he believed our understanding was beyond our earthly understanding.