So, it seems that pretty much every thread on here devolves into how irredeemably awful and broken beyond repair pathfinder is.
By and large, the people who decry it as "irredeemably awful and broken" are
not the people buying it in droves.
(I should note that that doesn't mean that the people who are buying consider it to be perfect. There's a middle ground there!)
So why is it the most successful product in the tabletop RPG world right now?
Several reasons.
Firstly, Paizo are, quite simply, a class act all around. They produce a wide range of products, all of very high production standard, and a great many of which are excellent all-around. Their customer service is the best of any company I've ever dealt with, period. They're very clearly a company run by gamers, for gamers, with a deep knowledge of, and love of, both the game and the literature that underpins it (that is, Appendix N, and its more recent equivalents).
But that's not all.
Paizo also got off to a flying start for (I think) three reasons: firstly, they inherited the database of Dragon subscribers, meaning they had a large initial audience, many of whom would at least
consider what they offered. Secondly, they benefitted from the anger WotC engendered when they cancelled the mags (whether that anger was deserved or not is irrelevant here - it was real, and it benefitted Paizo). And, thirdly, "Burnt Offerings" (Pathfinder #1) was a truly
exceptional product - they needed that first product to be a good one, and they
nailed it.
Paizo have also been smart enough to see the value of subscriptions, and also were in the fortunate position of being set up to handle subscriptions from Day One (thanks to Dragon). The advantage of subscriptions is huge - it gives confidence that the next product
will sell, it gives them a clear indication of how many copies to print, and it also means they don't need to market and sell each product individually (they only have to make sure their subscribers aren't motivated to quit). This, in turn, allows them to take some risks that another company wouldn't dare (such as "Rasputin Must Die!"), which further feeds into the virtuous circle they've got going on.
The short version: they had some advantages to start, they did a lot of things right, and they found ways to make their successes feed further success.