Plants vs. Zombies has been the most popular flash game on popcap.com, which is the #804 website in the US(1), for a while. iPhone gamers must have a large overlap with flash gamers.
Plants vs. Zombies was very popular before it ever hit the AppStore. And, PopCaps already had a massive fan base from Peggle and Bejeweled before P vs Z ever came out; they've been cranking out great stuff for a while now.
I have published a couple of games with PopCap. I think we were very fortunate to work with them. They have immense attention to detail and don't stop working on a game until it's perfect (at least for their in-house games - my own games were not as great).
So, PvZ is an awesome game to begin with, and the game-buying public knows they won't be disappointed by a PopCap game. Add to that a huge fan-base for the existing PC version...
PopCap is not afraid to throw away a game that's simply not excellent. They've developed a real reputation for quality, and they guard it really carefully. I've heard stories of games being developed in-house for months, only to end up entirely scrapping them in the end.
http://www.alexa.com/siteinfo/popcap.com