Because the changes in the alternate timeline occurred when she was an adult, and created a "new" history where Tasha served aboard the Enterprise longer and the war meant the Enterprise likely didn't perform some of the same missions it did in the proper timeline. To use my previous example, one of the problems with retconning the AoA into a parallel earth is that the choices of some characters were based on it being an altered 616 earth with a new timeline based on Legion's changes. Colossus, who was a rather unstable individual in the AoA, left most of his team to die at the hands of Apocalypse's forces because he didn't want Bishop to restore the "proper" history. If that occurred, his sister Magik (who never gained some of her normal powers in the AoA because she never encountered Belasco), would be dead from the Legacy Virus (which had already been unleashed by the time Legion tried to kill Magneto). Colossus wanted her to remain alive, and cared so little about other things that he accidentally killed Shadowcat when she tried to reason with him.
It's a fine line between arguing for a predestination paradox (and I see where MLB
is going with that) and arguing for an alternate future where many things are still conveniently similar, like most of the E-D crew still serving on the same ship in the same positions. It's a less radical shift than what's depicted in the AoA, but that wouldn't preclude the possibility of a character from that timeline ending up in the "normal" present after Bishop had restored things. That's essentially what happened with Blink's AoA counterpart, who was separated from her home reality to join the Exiles. Her 616 counterpart was only introduced later, and then for a short time before she was seemingly killed trying to stop the Phalanx. Only more recently did she reappear as one of Selene's followers, having become a vicious killer who believes the X-Men "abandoned" her to her fate and didn't try to save her.