Why does the T only go underground until Yerba Buena?

Does anyone know why, during the Central Subway project, they decided to go underground only until to Yerba Buena and didn’t include an underground stop at the Caltrain station?

The T train takes forever in that critical section. Even when the signal upgrades happen, that area should seriously be considered for an underground alignment to improve pedestrian safety, especially during events at Oracle Park, and to make the train more reliable.

Now with The Portal project underway, the N and T lines near the Caltrain station should also be put underground