Assume Facebook brings in $10/user/month in ad revenue.
But the distribution is uneven; $50/user/month for rich westerners, $6/user/month for poor people in poor countries who outnumber westerners 10:1.
If you offer everyone the chance to go ad-free for $10/month, your income from rich westerners drops from $50 to $10 and your total revenue drops to $6.36/user/month.
The users that are most valuable to advertisers are those with a lot of disposable income.
If you offer an ad-free experience for the value of the average user, far more of these high-income, high-value users would subscribe, both because they have the money and because they tend to be more privacy-conscious.
Imagine (simplified) losing ad income from the top 50% of your users for the price of your average user, while being left with only the bottom 50% to show ads to.