It's stupid, outdated, and misogynistic and I don't agree with it, but the reason is that teaching was seen as women's work, and they made the pay low as a barrier to entry for unmarried women. It's not meant to be the job of a breadwinner, the teacher is supposed to have a husband with a job that pays the bills so her income is extra. Wouldn't want those unmarried harlots living independently or teaching our precious children to have loose morals.