Actually is not THAT popular around here (Argentina) mainly beacuse they are not selling their boxed product, support is hard to get (no phone, no office, no forum) and things like that.
Most popular distro over here?
I'd say Slackware with Devian coming in in a second close (really close).
But the fact is that Brasil is the largest country in latin america, with more population than any other country over here and that in Brasil they DO sell a boxed product (with manuals and such), plus is a cheap OS in comparision with M$oft, it's way better than M$oft and support is easy to get.
A few years ago (3 o 4) Conectiva was selling their Boxed product over here, at a reasonable price and with manuals and enough documentation in SPANISH (which is a fundamental point to have in consideration when thinking why a distro is more popular than the other) like to get people interested on linux. And they did and that's why some of us still use Conectiva and not other distros.
Two points that should be mentioned are: pricing and availability.
Conectiva used to have those back then, but right now, a boxed version of conectiva is impossible to get around here (believe me, i've been looking everywhere)
Now: if Mandrake, Suse, Red Hat and others are also available in spanish, why going with Conectiva?
Maybe because it's latin or maybe because even though MDK, SUSE, or RH have low prices, those prices are still way to high because those prices are expresed in dollars and you have to add import taxes, shipping, etc...
The truth remains: non of them have support lines over here, most of them are really hard to get as boxed products with all the manuals, paying more than $40 (that would 140 argentinean pesos) for an OS just doesn't make any sense since we all know prices can be lowered just by producing those products over here (lowering production cost)
Have in consideration that around here SuSe's personal version costs $ 100 american ( $ 365 argentinean pesos) and that a normal wage around here is about $ 400 argentinean pesos a month... Now just do the math...
There are a lot of other issues but I think those would give you a picture why
if there's anything else you'd like to know about this topic or about how things work around here, just feel free to do it, i'll try to give a the best answer that I can find.