Why does Tom mention, for the second time, that the Joad’s weren’t people to write?

The first two people Tom meets, the driver and Casy, ask why there hasn't been any communication with his family. Thus, he feels as if he has to give them an explanation, even though it embarrasses him.

"Well, I ain't heard lately. I never was no hand to write, nor my old man neither." He added quickly, "But the both of us can, if we want."

Joad was embarrassed. "Well, Pa wasn't no hand to write for pretty, or to write for writin'. He'd sign up his name as nice as anybody, an' lick his pencil. But Pa never did write no letters. He always says what he couldn' tell a fella with his mouth wasn't worth leanin' on no pencil about."