$1.7 million is huge money indeed, but don't forget that most of big donations (50 or 100 BTC) were made long time ago when Bitcoin wasn't $11k. And I may be wrong, but I think that Bitcointalk have much more BTC in reserves. And big donations still happens, but in BTC it's that huge as it was almost decade ago. And I really don't know why people are still donating big money, when it was said multiple times that forum don't need donations to survive.
Sometime i feel that forum is missing a lot of funds by not using google ads. With the sort of traffic, this forum generates, google ads could generate a lot of revenue for the forum, but then it will not remain as clean (ads free) as it is now.
If you look to forum from business perspective, you're right. There is many ways how Bitcointalk can be monetised better. But I think Google ads is one of last things together with KYC which could be added to forum.
First reason why is privacy. I won't go into details because it's obvious.
Another thing is that you can't control content of advertisement. There is so many reports about scams and phishing websites advertised on Google. Probably it wouldn't be good to see such ads on Bitcointalk.
And finally, Bitcointalk isn't commercial business website which needs to maximize profits. Currently Bitcointalk have enough income to continue operations, pay to staff and for development of new software. There is no reason to look for extra income now.
Bitcointalk is one of rare websites which don't have million ads and you don't need ad blocker hera and I really appreciate that.