$\mathrm{LiF}$ is less soluble in water than $\mathrm{KF}$ because

$\mathrm{LiF}$ has higher enthalpy of hydration than $\mathrm{KF}$ 

$\mathrm{LiF}$ has higher lattice energy than $\mathrm{KF}$ 

$\mathrm{LiF}$ is more covalent than $\mathrm{KF}$ 

${\mathrm{Li}}^{+}$ ions are not extensively hydrated as ${\mathrm{K}}^{+}$