Why? 
Because I think - maybe I’m wrong - they are not two related (same root) languages. Czech is from Balto-Slavic root and Spanish (Portuguese) from Latin Western Romance root.
http://www.yourdictionary.com/library/pietable.html
A little different map here:
http://www.viking.no/e/heritage/indo.htm
Where Czech is Slavic and Portuguese/Spanish from Latin Italian.