Well, lot of different things - ideological, philosophical, spiritual, cultural etc. come into play.
For one, I avoid all allopathic medicines until I face some life threatening traumatic accident or the likes. I didn't need to take any allopathic medicines for last two decades. However, I did go through some X-rays and MRI for diagnostic purpose but for treatment I preferred Naturopathy.

I'm against harming any sentient creatures if there is an alternative available to me. Vaccines can't be made and tested without exploiting animals. According to some sources, Covid-19 vax uses even the blood of newborn (or unborn) calves.

Vaccine is not a medicine but some precautionary measure which is not foolproof in itself. Some one told me that it will reduce your chance of getting infected by 80%. This means, you're still vulnerable 20% of the time. On the contrary, non-vaccinated has a better stats in their favour. LOL! What are the chances that a non-vaccinated person will get infected by Covid-19 and die? Stats are a twisted way to visualize the truth. Over reliance on logic and science doesn't help.

Moreover, I want my body to develop antibodies on its own. I don't want any external assistance in the form of a vax as I have more faith on natural systems.

Other important point is that I ain't afraid of death. If I die of some disease, I'll die one day anyway! So let it be. I don't believe in taking a life insurance policy. I want to experience the life as it comes. Vaccine is akin to some insurance to me.

I also believe Vaccines may weaken the human lineage. According to Darwin, evolution favours stronger ones. So vaccine is against the Natural Selection of physically strong humans. For the good of few, larger humanity will suffer in the future.

In India, Naturopathy has been successful in curing thousands of covid-19 infected patients with a zero death incident. But there is no commercial interest in promoting it. So big pharma wins the war. Bill Gates will have the last laugh!

There are more such reasoning that would be difficult for me to share through this medium atm. But you get the idea.
So to each her own. That's why it shouldn't be mandated. I'm against vaccine passports. But my views hardly matters to authorities. So I should keep those to myself :)