It's just that ads is significantly more profitable than having to run sales teams in the 200 odd countries around the world. Especially since ads is largely self service even down to ML models monitoring abuse.
Facebook already having an ad business can also be used as a counter-argument: cannibalization. Some of the ad-spending going to WhatsApp would definitely be spent on Facebook instead if WhatsApp remained ad-free. But I can't imagine that they are not fully aware of this (even Hanlon's razer must have limits somewhere), so what remains is that they consider the expansion of addressable ad audience more valuable than other monetization strategies plus the massive hit the WhatsApp brand will take from ad introduction. And because those ads would be almost worthless without the targeting ad customers expect from the Facebook brand, the whole thing is like an indirect confession about the existence and quality of shadow profiles describing those who are not registered with the mothership. Well, that group is about to shrink...
Advertising is charging for the fantasy of more business. Oddly enough, the production facilities for fantasy scale with the customer count. ;)
Sales teams all over the world is expensive, self service ads supported by a small team are a lot cheaper.
E.g. A certain telecom api provider doesn't need sales teams all over the world. In fact I've never seen any ads or anything about them locally, but guess who's on top of my mind when I need to send or receive sms in some other country?