I have a feeling there was a lot of internal push back and fourth, but they were either losing too much money or only getting advertisers they weren't comfortable with.
It's more complicated that just advertisers expectations.
https://www.fastcompany.com/90304153/seven-weeks-after-nsfw-...
If the only way to save your platform is to reform society, then your platform is already dead.
In the long run though, I expect reddit to ban porn. It might be a few more years, there are a lot of factors in play, including the patience of their investors and whether or not an influential media company decides to do an exposé on reddit hosting amateur pornography without vetting peoples' ages and/or hosting professional pornography in violation of copyright law.