Gothic cathedrals contained stained-glass programs primarily to

Gothic cathedrals contained stained-glass programs primarily to tell Bible stories to a most illiterate audience.

Given that at the time, most of the people couldn't read, as books weren't really a thing back then, they still had to be educated about religion and biblical events somehow. This is why scenes from the Bible were depicted on these programs so as to educate people who couldn't read as to what Bible is all about.