edit: people seems to miss that I said that this is only part of the reason not the reason. I also said many places outside the US (not all). So do people down vote because they think that there are only US and EU in the world? I am missing that point.
Even the cheapest of plans from the main networks (3, vodafone, EE, o2) - as low as £4/mo have unlimited calls & SMS now.
edit: this has been the case for quite a few years now, too.
It is explicitly designed to give carriers back control of messaging.
Why am I even saying this? We all remember how much fun carriers control over SMS was. It killed sms. There were never any new features. Cost made lots of stuff, such as multi party chats effectively impossible for teenagers ... It was always very unclear how much everything cost. There were "traps" with premium services, often not advertised as such. Now there's fraud with international SMSes. And things have gotten worse' several states made clear they use phones to find abortions ... And RCS is designed to bring this back. No.
Just. NO.
That changed, but e.g. with my pre-paid plan I still pay 12 cents per SMS.
Historically there was also a problem that unlimited SMS plans worked only within a country. Now it's a bit better being EU-wide, but still not worry free.
My 3.99€/month plan includes 100 SMS. Not that I ever use all of those, all of my 100 minutes, or all of my 1 GB of data.
No spam because you needed SMS text verification, no ads, no login to remember, just all around amazing app at the right place at the right time.
The only thing I'll add is that, ca 2012 which is when whatsapp started really taking off where I live, intercarrier SMS was slow and unreliable. WhatsApp was fast and reliable.