More can be written, but you might like to start with these (which give similar answers)"

• "Why didn't Nazi Germany kill all the people in concentration camps before their collapse?" by /u/kieslowskifan
• "Some Holocaust survivors were moved from camp to camp. Why did the Nazis spend all that time moving them around? Why not simply kill them?" by /u/coinsinmyrocket

These appear to be the most applicable from the FAQ section "Holocaust and Nazi Crimes Against Humanity".

This is not to discourage discussion. More questions, data and debate are welcome.