The factor responsible for weak acidic nature of B-F bonds in BF₃ is

BF₃ is electron-deficient because B only has six valence electrons. B has one empty 2p orbital, while each F has three lone pairs of electrons. B and F have similar sizes, as a result, B and F orbitals show effective overlapping. Therefore, F can share one of its lone pairs with the unoccupied orbital of B. This results in the formation of $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding. This provides additional strength to the B-F bond and makes the B-F bond less acidic.