Because of the strong H-F bond.
What makes an acid a strong acid is that the bond is weak and/or polar. This makes it easy for the hydrogen to come off. When there are many
Initially, it seems that HF should be a strong acid because the other elements in that column make strong acids (HI, HCl, HBr) and also because Fluorine is the most electronegative element.
However, the bond between hydrogen and fluorine is very, very strong. Fluorine does not want to let go of that hydrogen. For this reason, HF is a weak acid.