What was one important factor that pass the 26th amendment in 1971?

The Vietnam War. It was a long debate over lowering the voting age from 21 to 18, which began during World War II and only intensified during the Vietnam War when young men who were practically being heavily obligated and sometimes forced/drafted to fight for their country were being denied the right to vote. 
“Old enough to fight, old enough to vote” became a common slogan for a youth voting rights movement, and in 1943 Georgia became the first state to lower its voting age in state and local elections from 21 to 18.