I see two reasons for this. First is a carryover of distrust from the mishandling of the pandemic up to this point. There has been flip-flopping in government messaging and policy constantly. This has sometimes been due to new data, and sometimes I'd argue for political gain.

The second reason is, well, the free discussion of the nuances of the vaccine has largely not been allowed/socially acceptable. The vaccine is being pushed in very forceful way while honest (if not always scientifically accurate) discussion is repressed. Although there will certainly be a group too far gone to ever accept the vaccine, this strategy of a heavy handed and propaganda-like approach seems to be causing the vaccine hesitant to only dig in their heels further. The vaccine can't win in the marketplace of ideas if the marketplace is not allowed to exist.