They have a history of being very picky about allowing certain apps that compete with their core products.

I think what you're talking about is just that: history. There is a slew of email, weather, web browsers etc. All in the App Store competing with their native versions. There have been for a while.

They also have a more recent history of literally promoting "competing" maps applications in the App Store after a botched launch of Apple Maps.

Google Voice for example. Apple felt that another app having a dialer would be confusing to users. I'm not sure what excuse they'd have to block Google Maps, given the large selection of mapping apps available, but I wouldn't put it past them to come up with something.

Google Voice is in the App Store these days. It's pretty clear that they've worked to come up with a fairer and more consistent set of approval guidelines.

Two things:

1. Google now gets stats from millions of iPhones. Which can be used to infer important sales information in different regions.

2. Maps improve as more people start using them. If more users are on Google Maps, Apple's own product improves at a slower pace.