Boron and thallium belong to group $13$ of the periodic table.

In this group, the $+1$ oxidation state becomes more stable on moving down the group. $\mathrm{BCl}_{3}$ is more stable than $\mathrm{TICl}_{3}$ because the $+3$ oxidation state of $\mathrm{B}$ is more stable than the $+3$ oxidation state of $\mathrm{Tl}$. In $\mathrm{Tl}$, the $+3$ state is highly oxidising and it reverts back to the more stable $+1$ state.