Perhaps survivorship bias explain part of it. Many of those scientists arrived at US fleeing their home countries in WWII. Migrating under those circumstances is tricky, lacking the necessary support network or failing to successfully negotiate an encounter with border patrol may be the difference between reaching a safe place and being dragged to rot in the trenches. Good verbal intelligence is very helpful in this process. Perhaps the same way good verbal intelligence is over-represented in those who escaped the war, the trait of being taciturn was over-represented in all the scientists that got stuck and died in WWII[1].

[1] https://worldscientific.com/worldscibooks/10.1142/q0436#t=ab...