Why is latent heat of vaporization not exactly proportional to boiling point?
Hm, the proportionality is not so very good. (data from the wikipedia link above)

The line is a linear regression through all ten data points.

The answer to your question is given on the same wp article. The quotient of enthalpy of vaporisation and temperature is the difference in entropy between the liquid and gaseous phase.

According to Trouton´s rule, it is about 85 to 88 J/molK (ca. $10.5 R$) for most liquids. https://en.wikipedia.org/wiki/Trouton%27s_rule explains the reasons for deviations, like hydrogen bonding, small rotational excitation, dimerisation in the gas phase etc.