Because it's a new vaccine and the mRNA ones are a completely new type of vaccine. They seem safe in limited use over a short period of time, but we don't know

Because it's a new vaccine and the mRNA ones are a completely new type of vaccine. They seem safe in limited use over a short period of time, but we don't know yet what effects they may have in the future. Also there is an aspect of if you are young and healthy your risk of serious effects from covid is low. Even if your risk of problems from the vaccine is low, if your risk of serious effects from the disease that the vaccine treats is also very low then why bother taking any action.