Two issues mentioned in this npr report:
- The workers might think that union dues cut into their wages.
- The workers might think that they can negotiate a better deal for themselves without an union.
Note that these were raised by Amazon itself. How genuine employees are when they repeat them is hard to tell.
Additionally, from the end of this cnbc report:
- The workers might think that unionization makes the company more unwieldly and thus ultimately cut into their own career prospects.
The second bullet point above seems to be quite common in highly paid, highly sought-after professions. When the threat of the employee resigning is an actual threat to the employer, unions appear less necessary.
Finally, and I don't know if that was raised in the Alabama case, workers might fear that unionizing will cause the employer to shut a site down, fire all employees, and open a new site where there are no unions. One might call that blackmail, and it doesn't have to be spelled out in detail to change opinions. This BBC report mentions fear by city officials that the city would get less attractive to businesses, which is a similar statement.