From the BBC
The instruments began displaying different speed readings, probably because of iced speed sensors while their heating systems were shut off, the committee said....
Russian media reports said the plane's captain had rejected a de-icing treatment on the plane before takeoff. The procedure is optional and the crew's decision is based mainly on the weather conditions.
Russia Saratov crash: Ice on sensors 'may be cause' - BBC News
De icing is typically wings and tail, yes ? The pitot ports are electrically heated.
Though as it was snowing at the time on the ground the lack of de-icing is still maybe notable.