My question isn't "why?" It's "why not 5/$70?"
Who else was going to try to sign him away? Why pay more if you don't have to?
The only thoughts are;
1) Perhaps Sacramento called early in the process which led to negotiation. A max from SAC would have been 4 years/$68 mil -- maybe they were a viable enough threat at one point during the process.
2) They went down the hardline negotiation road with Monroe last year, and he walked and DET got nothing.
3) Related to 2) -- they just traded for Jackson, albeit at the not huge cost of two 2nds and Kyle Singler, and didn't want to lose him for nothing next year. It's easy to talk yourself into overpaying after already having made an investment, because sunk costs are hard to ignore, even if they are already indeed sunk.