Because it wouldn't make any grammatical sense. Koń is masculine, so it's "ten koń" and here "tego konia" because "pamiętać" takes accusative, and even if the noun was feminine, the accusative would be "tę", like "Pamiętasz tę regułę?" - "Do you remember this rule?"

Watch out: maaaaaany Polish people would incorrectly use "tą" in the sentence I just gave, especially in speech. It is incorrect though, the accusative form of "ta" pronoun has to be "tę".