Pyridine is less basic than triethylamine because:

The basicity of amines depends on the availability of the lone pair of electrons on the nitrogen atom.In triethylamine, the nitrogen is $s{p}^3$ hybridized and the lone pair is localized.In pyridine, the nitrogen is $s{p}^2$ hybridized. Since $s{p}^2$ orbitals have more $s$ -character than $s{p}^3$ orbitals, they hold electrons more tightly.Most importantly, in pyridine, although the lone pair is in an $s{p}^2$ orbital and not part of the aromatic $\pi$ system, it is still less available due to the delocalization of the surrounding aromatic system or the delocalization of the nitrogen lone pair itself within the ring.