I got two options,
- The size of $\ce{B^3+}$ ions, is very small and the ion has high charge density. Thus, following Fajan's rule, boron forms covalent compounds.
- The sum of the first three ionisation enthalpies of boron is too high, thereby restricting the formation of $\ce{B^3+}$ ions. Thus, boron is forced to form covalent bonds with other compounds.
Which one is a better explanation?