So much of the platforms of Bernie Sanders and Elizabeth Warren require total control of the houses of Congress to the point that a majority of what they're campaigning on is a non-starter in terms of ability to be implemented. Theres no way that Universal Healthcare or massive gun control measures will pass the Senate with the needed 60 votes because the GOP will not compromise on either of those issues (at least to any degree Warren or Sanders voters would be satisfied with) and the Dems wont win enough Senate seats to give the party a super majority to pass whatever legislation they want.
A large number of the states with Senate races in 2020 are contests where Dems simply dont have a good chance of winning.... Arkansas, Alaska, Georgia, Iowa, Kansas, Kentucky, Mississippi, Tennessee.... Most races take place in states that are so inherently Republican that no Democrat has a feasible chance of flipping the seat..... Some organizations think that there are only 4 seats where the seat can be flipped, and one of them is a Democrat seat at risk of flipping back to the GOP:
Arizona - GOP
Colorado - GOP
Maine - GOP
Alabama - DEM
Assuming that Senate seat in Alabama flips back to being Republican, if the Dems win in Arizona AND Colorado AND Maine, then that results in the Senate only swinging two more seats into the favor of the Democrats, putting the total at 51-49 for the Dems (This is counting 2 Independents who usually caucus with the Dems)..... Thats simply not enough of a majority to ram major legislation like gun control or universal healthcare through Congress in order for it to be signed by the President.
Whether the policies proposed in the first place is sensible or too far to the left is one thing, the simply math in the Senate that needs to be done for there to even be a chance of the legislation being passed is something else entirely. A majority of their platforms simply cannot be enacted without some sort of massive voter shift in GOP stronghold states flipping the senate seats to the other side, meaning that a majority of their platforms are unobtainable fantasy, regardless of the merits or motivations behind them.