To be honest, nobody knows the truth, she 's changed since the Equestria Girls Forgotten Friendship, and then in season 8 and 9 and the EG shorts, maybe she was busy with other works, or worse, she got tired of Pinkie Pie, but this will be horrible, but it happened sometimes, as example, Italian Conan of Detective Conan (the child version) has changed voice because the girl who gave the voice had enough of the character, so she was replaced with another voice actress, but this last one died last year, now we need another voice for little Conan. Something happened with Rarity, her voice actress was replaced (Italian Rarity it's the same italian voice of Tori Vega in Victorious) but in this case she had to leave the dub of MLP because she was pregnant, then she is back in the other seasons, Rarity has her right voice in season 8 and 9 and Forgotten Friendship, but not Pinkie, I hope one day we will discover the truth.