It is a general rule that ionic compounds are good electrolytes, but this is because most ionic compounds completely dissolve into ions when placed in water. For example, outside of water solid sodium chloride salt (NaCl) is a non-electrolyte because the ions are all locked up in the Na-Cl bonds and can't move. In water, the salt dissolves into separate Na+ and Cl- ions. These ions can now move around separately from one another in the water and in doing so they can conduct electricity.
In contrast the H-F bond is so strong that water cannot break the H-F bond anywhere near as well as it can break the Na-Cl bond. So when you place HF in solution it largely doesn't break apart into separate H+ and F- ions; it still stays HF for the most part and HF doesn't conduct electricity. Only the H+ and F- ions are capable of providing electricity, and, since you only get a few of these ions forming, HF is considered a weak electrolyte.