Pyridine is less basic than triethylamine because

In pyridine N is sp² hybridized. Electronegativity of sp² hybridized N-atom is more and lone pair electrons are strongly held by N-atom. These electrons are not readily available for donation. In trimethyl amine due to +I -effect of three CH₃ groups, lone pair electrons are readily available for protonation. (CH₃)₃N is more basic than pyridine.