It might be a business decision. It might be a convenience decision. It really depends; the only thing you can say for sure is that it's the developer's decision.
You're assuming that "iPhone and Android are sending the data to a single service," which isn't always the case. The iOS APIs for leaderboards (Game Center) send data to Apple. Google's Leadersboards service sends data to Google. A developer may choose to use the platform-provided leaderboard API for his or her game because it's easier or cheaper for them (despite the fact that they'll need to build an implementation against both APIs). They may also choose to do it because those APIs offer better device integration than 3rd party APIs might, and they feel this creates a better experience for the player.
There are various third-party leaderboard APIs a game might adopt. Some of these may have additional cost (either as retail products or in the form of requiring the developer to provide and pay for the hosting of the leaderboard databases). They may not look or feel as integrated with the device, either. Those potential disadvantages have to be weighed against the advantage of only having to write against the API once (in theory), reducing development time.
The particular balance of all those factors (and more) will all play into the decision of which particular method to use for leaderboards; it basically depends on the developer's needs and wants.