Why did God react so differently concerning the lack of circumcision in Exodus 4:24-26 and Joshua 5:2?

[tonychanyt]

God wanted to kill Moses for not circumcising his son in Exodus 4:

24 At a lodging place on the way, the Lord met Moses and was about to kill him. 25But Zipporah took a flint knife, cut off her son’s foreskin and touched Moses’ feet with it. “Surely you are a bridegroom of blood to me,” she said. 26So the Lord let him alone. (At that time she said “bridegroom of blood,” referring to circumcision.)

Why didn't God just order Moses to circumcise his son?

Moses knew the statute and should have followed it already. God told Abraham in Genesis 17:

10 This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised.

Now, on the other hand, Joshua 5:

2 At that time the Lord said to Joshua, “Make flint knives and circumcise the Israelites again.” 3So Joshua made flint knives and circumcised the Israelites at Gibeath Haaraloth.

How to explain the difference in God's reactions to non-circumcision in this case?

It wasn't their fault. It was their fathers' fault and they died already.

See also Why were the Israelites not circumcised in the wilderness?.