Also, you can say that the API level differences don't matter because they are hidden by compatibility libraries like The Support Package, but the truth is that those are just the newer APIs re-implemented against the old interfaces. You pay for that extra abstraction with wasted cycles and bloat, and it's an ugly solution. Even still, those libraries aren't perfect: there have been subtle bugs or incompatibilities introduced by their use which are notoriously hard to find. Finally, even if the newer APIs are present on the device, The Support Library will still be used (last I checked, though I think they are working on this).
Less objectively, it's a matter of preference. Some people just prefer objective C to a JVM compilable language.