The Hayashi track has an almost constant effective temperature. The pre main sequence star (not protostar) is powered by the release of gravitational potential energy caused by its contraction.
If the star is getting smaller at constant effective temperature, then of course it's luminosity ($propto R^2 T^4$) decreases.
So perhaps your question is why do PMS stars contract at constant temperature? In other words, why is the Hayashi track almost vertical in the HR diagram? The answer to this is derived in some detail on the relevant Wikipedia page and arises from the fact that low mass PMS stars are fully convective and have atmospheric opacity dominated by H- ions.
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