The home must meet FHA guidelines in order for it to qualify, so that may be a consideration of the listing agent. The agent may know it does not meet all of the guidelines. There is also a cap in every county of the loan amount for FHA mortgages.
I would ask your agent to find out from the listing agent believes this home does not qualify for FHA financing.
Some agents and sellers are just wary of FHA loans because they used to more difficult than a conventional mortgage. In addition, the seller would incur extra expenses. In most cases this is no longer true.
They actually are pretty easy ( in most cases) and offer a good interest rate and little money down . I believe that they now require 3.5% down payment. Difficult to find many other mortgages like that today!
The best thing to do is to simply find out from the listing agent why. The seller does not have to accept the offer, even full price if they did not agree to FHA financing in the listing agreement.
Good Luck in your purchase