This is because Obama has done something that Left Twitter never has - he's won a national election.Calls out left twitter.
Like most political extremists, Leftists in the US are very loud online and thus have a distinct tendency to start drinking their own Kool-Aid and believe that they represent some kind of significant voting block - which is not the case most of the time and is really not the case when it comes to Presidential elections. Warren is already going to have a very hard time winning Pennsylvania and Florida. If she chains herself to the far left in order to win the primary, then she's almost certainly going to sink in the general.
Of course, it's normal to veer outward for the primary and then turn towards the center once you secure the nomination, but you do have to be careful about over-extending. Unless you're Trump, of course, but there's only one of those.