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.