Most of the details about the Ewell family are given in chapters 17 and 18, which are trial scenes. Bob Ewell's testimony is given in the second half of Chapter 17, and Mayella's in Chapter 18. In neither of these chapters are we told the name of Mayella's mother, nor any details about Bob's wife's death, such as how long ago it took place, what she died of, or what happened after.
Though we never learn the name of the hapless, now deceased Mrs. Ewell, it is easy to infer why her husband did not remarry.
Mayella Ewell is the oldest of seven children. She is nineteen years old. Assuming the children are spaced about two years apart, that would make the youngest seven years old. So we know that Mrs. Ewell died no more than seven years ago, possibly more recently than that. That would make Mayella about twelve at the time of the mother's death.
We know that Mayella went to school only two or three years. By age twelve she was probably finished with school. We know from descriptions given by Scout (as narrator), Tom Robinson, and Mayella herself that Mayella was the one who essentially kept the Ewell household running. As Tom Robinson says in Chapter 19,
"Well, I says it looked like they [the rest of the family] never help her none ... I felt right sorry for her, she seemed to try more'n the rest of 'em ..."
Probably, by the time her mother died, Mayella was already running the household. Certainly after she died, Mayella took over. She did the chores, she kept geraniums, she scraped and saved money to feed her siblings when her father drank up his welfare cheque. With Mayella around, Bob Ewell did not need a wife.
Even if Bob Ewell had been on the lookout for a woman willing to marry him, it's hard to imagine how he could have found one. He brought in no income and was a drunkard. His children lived on poached game and whatever they could find in the dump. It's hard to imagine any woman marrying such a man voluntarily.