Intramolecular H-bonding requires the donor (-OH) and acceptor (-NO${}_2$) to be close enough on the same molecule. In $o$-nitrophenol the two groups are on adjacent carbons and form a six-membered chelate ring through an O-H$\cdots$O bond. In $m$- and $p$-isomers the groups are too far apart, and HF can only form intermolecular H-bonds.