It could just be that Amazon is not able to fulfill this product with Prime shipping. Instead, look to see which sellers are "Ships From Amazon", as those are the FBA sellers who will most likely be prioritized in the buy box.
If there are no FBA Sellers, then it could mean you are not allowed to list this product as FBA.
*If your ship to location on Amazon (your consumer account that is) is set to somewhere Canada, you will most likely not see prime either. If this is the case, look up an address in the US you can put as your temporary address when you do product research.
Why is there no prime filter?
How do I turn on the prime filter?