ergo vs that's why

ergo

adv
  • Consequently, therefore, or thus. 

noun
  • An ergonomic factor or characteristic. 

conj
  • therefore (especially in syllogisms) 

that's why

adv
  • a statement to emphasise the reason or explanation just given 

How often have the words ergo and that's why occurred in a corpus of books? (source: Google Ngram Viewer )