It's the most popular in the USA. In different areas of Europe, other decks were very popular and probably most popular, but in the last years, the high quality books and RWS-inspired decks has pushed the RWS to the fore everywhere.
I remember that 20, 30 years ago, in Germany THE deck was the Thoth. But I have only anecdotal evidence and can't really prove it.
The RWS is a good deck but IMO the Thoth is at least as interesting and more challenging. With the Thoth, you have to get along without the "narrative crutches" and deal with complex esoteric associations. If you dig deeper into the RWS, you will find also there a very complex system but the entrance is easier because you have the scenes and stories to hold on to.
While it is possible to read the Thoth just by looking at the images - the art is strong enough to help you along - you need to learn about astrology, alchemy and kabbalah to get the most of it. That's enticing to some readers, intimidating to others. The RWS is more reassuring to a beginner.
It's like the difference between figurative and abstract art. Everybody can "judge" a figurative painting. People look at it and say, "okay, it's supposed to be horse, so how successful does it convey a horse?" With abstract art, you really have to go beyond that, and that's why many people belittle it and say, "oh well, anyone can make some colourful blotches, but does he know how to paint a horse?"
I personally think it's a pity that the RWS has taken over the popular imagination as "real" tarot and that people are disturbed if a new deck doesn't fit into their RWS-based notion on how the card has to look. It's one of my pet peeves when people call RWS images "tradtional". They're actually not that traditional; they do signify the beginning of the RWS tradition, one among many.