According to a team spokesperson to RaceFans, they decided to retire Kubica to conserve parts. Their reason for retiring was that it made no sense to continue as they were not in contention for points.
With regard to Russell’s crash, Williams F1 is investigating the cause of the lock-up which sent his car into the barriers at turn eight.
According to Kubica’s engineer Dave Robson, “We’re not in an easy situation and Japan is right there.”
Given Williams lack of competitiveness in 2019, Kubica’s retirement is likely owing to a worry that the unexplained incident for Russell could happen on the second FW42.
“This weekend has been very disappointing.” said a disappointed Kubica. “You always have to try your best with what you have, and this is what I was trying to do. After George’s DNF, the team decided to stop my car, to take care of the parts that we have on the car. We are not in an easy situation and Japan is around the corner.”
The double retirement is the team’s first double DNF in 2019, a week after their first retirement of the year with Russell in Singapore.
It is also worth noting that given their troubles at the start of the season, there is a serious lack of spare parts. As a result, they have to be extremely conservative and hope and pray that their retirements and crashes are kept to a minimum.