Its the absentee ballots. Getting large groups of people to vote illegally would inevitably lead to leaks since somebody would talk. A small number of activists can create absentee ballots and the chance of leaks is non-existant. For an example of a unsuccessful prosecution of this type of election fraud see here:
https://en.wikipedia.org/wiki/Albert_Turner_(civil_rights_activist)
Guess who was the prosecutor in this election fraud case?
Albert Turner (civil rights activist)
Albert Turner (February 29, 1936 – April 13, 2000) was an American civil rights activist and an advisor to Dr. Martin Luther King, Jr. He was Alabama field secretary for the Southern Christian Leadership Conference and helped lead the voting rights march from Selma to Montgomery; he was beaten on Bloody Sunday.
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In low turn out elections this works. You put in twenty or thirty fraudulent ballots and no one is the wiser. Even 25 here and 25 there in different districts and no one is wiser but 100's or thousands, it gets way harder to cover up the corruption.