My answer is based on the article: "Selecting a Landing Site for the ExoMars 2018 Mission".
Vago, J.L., Lorenzoni, L., Calantropio, F. et al. Sol Syst Res (2015) 49: 538. https://doi.org/10.1134/S0038094615070205
In section 5 "LANDING SITE’S ENGINEERING CONSTRAINTS" the authors provide a list of the different constraints: Latitude, Elevation, Landing ellipse, Landing ellipse azimuth and more. In this list, it is mentioned that: "For a 2018 launch, the landing ellipse azimuth can be between 90° and 102°. For a 2020 (backup) launch, the landing ellipse azimuth can vary from 113°–127° (at the beginning of the launch window) to 88°–103° (at the end of the launch window).
"Landing ellipse azimuth: The landing ellipse azimuth will change depending on when the mission is launched during the window (usually two weeks long).
In the case of ExoMars 2018, this change is more manifest for the 2020 backup launch than for the regular launch in 2018."
It is important to note that the article was published in 2015 and since then the project may have added design constraints.
In a different paper (ExoMars Landing Site Characterisation and Selection), the authors mention another difference between the 2018 and the 2020 launch.
"For a 2018 launch, Oxia Planum is the leading candidate landing site and its site characteristics e.g. rock abundance , aeolian cover and predicted atmospheric conditions, are currently being checked. For a 2020 launch, Oxia Planum is one of the two candidate landing sites, with a second to be selected from Aram Dorsum and Mawrth Vallis during 2016."
It seems that the 2018 launch offer just one interesting landing site while the 2020 offer two candidates.