I don't think that's true. Tulsi Gabbard is probably the most objectively disliked of the 2020 contenders so far.
The title of this thread should be: ''Why are Sanders and Warren the most disliked by people who obsessively follow politics?''
The actual voters who determine the outcome of elections dont feel all that strongly about any of the candidates and just vote for short term circumstantial reasons
That's definitely true...but even among obsessive politics followers; Gabbard, of the already declared candidates, is still probably the most disliked overall. There are a sizable number of posters on this forum that seem to be skeptical of almost every candidate, but Gabbard's dissenters seem to transcend ideology.