In the millennial kingdom, animal sacrifices for sin will start again at the new temple there, from what I understand. Why is that?
The Bible never explains why the sacrificial system is resumed during the Millennial Kingdom. However, there is one thing we do know about animal sacrifices...they cannot take away sins. So, we know they won't be done for that purpose. Hebrews 10:11 says:
Day after day every priest stands and performs his religious duties; again and again he offers the same sacrifices, which can never take away sins.
The fact is, the sacrificial system never took away anyone's sins. It simply deferred the punishment temporarily, showing the worshiper that another sacrifice was necessary. Ultimately Jesus fulfilled that sacrifice by His own death. His death was necessary because the sacrificial system left the sins of mankind unpunished. (Animals cannot stand in for humans in order to gain freedom from sin.) As Paul wrote in Romans chapter 3:
God presented [Christ] as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished--[3:25]
So, if the sacrificial system didn't take away sins under the old covenant, they certainly won't be able to do so during the new covenant either. So our best bet as to why they are resumed in the Millennial Kingdom is that it will be a kind of memorial--much like Communion for the Church is observed "in remembrance" of Christ's sacrificial death on the cross.