What are the unthinkable (acinteyya) dhamma?
The reason is given in the sutta itself. Pondering on these brings madness and frustration for someone with an undeveloped mind. 


  "These are the four unconjecturables that are not to be conjectured
  about, that would bring madness &amp; vexation to anyone who conjectured
  about them."
  http://www.accesstoinsight.org/tipitaka/an/an04/an04.077.than.html#fnt-1