When the ‘Medicare for All’ plan was revealed in 2017 by Sanders and updates this April 2020 contenders & four of his colleagues joined him both times: Kirsten Gillibrand, Elizabeth Warren, Sens. Kamala Harris, Cory Booker. At the time, they wanted to co-opt an idea which already had a strong following and to not allow room on their left on a dominant issue of the campaign.
However, by taking this approach, not only did they unnecessarily linked themselves with the most extreme position on healthcare, but they gave themselves considerably less flexibility for the articulation of their own vision on the issue during the campaign. In particular, it meant that they endorsed a plan that would efficiently eliminate private insurance by: transfer of all Americans into a single government plan within 4 years, by outlawing all the private plans which cover any of the same benefits as the government plan and lastly by offering such a vast range of benefits on the government plan, that it would effectively leave nothing for private plans to cover. At the same time, Sanders, who is the lead author of the bill, has frequently argued that it would get rid of private insurance with the exception of cosmetic surgery coverage.
There was no point for these candidates to make Sanders their healthcare leader.
When the Sanders plan came out, rather than joining as a co-sponsor, Warren could have said something to the effect of that he does support the idea of Medicare, but he will be release his own plan outlining his own vision on healthcare.
Sure, anything is possible in a general election matchup against Trump. However, by allowing Sanders set the terms of debate on healthcare, his opponents created a pointless liability for themselves by ensuring that the fight over universal healthcare would be fought on the terms which would be least favorable to them.