Join Russian-European Mars mission was officially moved to 2020 due the decision of Johann Woerner and Igor Komarov.
Previous gossips about delays in program were unveiled in March 2016, just before successful launch of Schiaparelli EDM and TGO space probe on atop of Proton rocket on March 14, 2016 (You can read more here about launch and here about statement of Igor Komarov). Chief of Roscosmos Igor Komarov said in the interview given to TASS news agency:
"The issue of the mission’s possible postponement to 2020 is being discussed but the final decision has not yet been taken... It is the project’s phase two and we are not yet through with its discussion with the European Space Agency,"
It seems that discussion with ESA took place and both organizations reached same conclusion - mission should be postponed to next launch window (it seems that Mars missions have bad luck - Insight was also postponed for two years from 2016 to 2018). Second mission under ExoMars was based on Russian lander and rover designed and manufactured by European partners, so both sides are responsible for keeping schedule equally. In official ESA statement on further plans for ExoMars we could read that in 2015 special joint group of Roscosmos and ESA specialists along with representatives of space industry was established already in 2015 to recognize situation and provide an assessment of possibilities to keep original schedule. Unfortunately Tiger (this is how group was named) presented not quite good conclusions - delays in program on both sides are too big to keep 2018 as launch date and start of the next mission should be postponed. ExoMars still remains important for Roscosmos and ESA, but ESA director Johann Woerner and Roscosmos directors Igor Komarov decided to move mission to July 2020. Both directors decided also about creating new schedule with target date of 202 and improve internal control on the project to avoid further delays.
On picture above: ExoMars lander by NPO Lavochkin.