American politics have become divisive. Many want the the Democratic candidate to represent both sides.
Buttigieg is unique to other Democratic candidates: he is the only LGBT and the youngest candidate. He represents a new generation, and much of his platform is based on the idea of bringing about generational change. At the same time, Buttigieg has a lot of qualities that appeal to more traditional Democrat voters. He is a military veteran with more centric policies than most of the other candidates. He cut his teeth in elected office in the fourth-largest city in Indiana. In a time when American politics is extremely divisive and full of in-fighting, Buttigieg synthesises the old and the new. The Democrats do not need a candidate at the extreme end of the political spectrum - they need someone like Buttigieg, who will unite voters.
While Buttigieg certainly has qualities of a 'traditional' Democrat, he does not appeal enough to further Left voters. Being young and gay does not mean he is necessarily in touch with voters of all interest groups, nor does it mean he necessarily offers innovative policies. He does not represent what many of his generational contemporaries see as the future of the Left, prompting reactions from derision to overt hatred. While ending divisiveness may appeal to some, to others it is not appropriate to compromise in a time when sweeping policy change is needed. Buttigieg is ultimately a centrist candidate, in a time when many voters are steadfastly rejecting centrism.
[P1] Buttigieg synthesises qualities of traditional Democrats and the new, rising Left. [P2] This reflects the qualities America needs in a President to stop the increasing divisiveness.
Rejecting the premises
[Rejecting P1] Buttigieg does not appeal enough to the rising amount of left-wing voters that favour candidates like Warren and Sanders.