1) He was the central figure in engineering the device.
2) He understood exactly what he was doing and made repeated efforts to perpetuate the lie to the government and EPA.
3) He led efforts to damage control and roll out an update to consumers under the guise of improved performance, but really to cover his own ass when the software got stuck in "testing mode" and deteriorated engine components.
4) He lied to the US government when confronted and led efforts with conspirators to bury.
I am going to guess that his relatively short 5-year plea also comes at the price of turning state's evidence on his often mention "co-conspirators".
I'd say it's probably a little preliminary for us to assume that only a single engineer is going to wind up behind bars for this, just because he's first in doesn't mean he's last out!
More data here:
https://www.justice.gov/opa/pr/volkswagen-engineer-pleads-gu...