“Why did God wait so long to send humans (and only humans) a savior?”
Might I recommend “Out of the Silent Planet” by C.S. Lewis?
http://www.amazon.com/Out-Silent-Planet-Space-Trilogy/dp/0743234901/ref=tmm_pap_title_0?ie=UTF8&qid=1368310330&sr=8-1
God exists outside our understanding of time...so the question as to why God “waited so long” is improper defacto.