Bitcointalk.org is primarily a discussion forum, not a business ran to maximize income. There are many reasons against having Google ads, such as: privacy, preferring to have more control over which ads are shown on the page, and not wanting to censor more content than necessary just to please Google (to be compliant with their gambling policies, among others).
Start using Google Ads and scammers will use it to advertise phishing websites pretending to be Ledger and Trezor. Search "Ledger" on Google and sometimes you will get a scam website asking for your seed. We don't need Newbies coming here, saying that they got scammed by an ad they saw on the forum.
Google sucks. At least theymos knows what he is doing when he accepts someone's banner in the ad rounds.