A passcode you can change, virtually an unlimited number of times. A passcode is also digital, in other words, if you don't enter it exactly right, it's not going to work.
A fingerprint is both limited, you've got 10 of them, and analog, which means your iPhone checks to see if the object pressed against the sensor is close enough to the digital signature on file. That's why a fake finger works as well as a real finger, and why you can use your finger in a variety of positions on the sensor and variety of "smooshedness" on the sensor. In reality, the sensor calculates a probability that it's looking at the correct finger, and then decides to unlock, or not, based on that probability function. An infinite variation of fingers and finger-like objects will unlock your iPhone. The "size" of that infinity is not as big as the "size" of infinity of fingers and finger-like objects in the world, but it's not negligible, and that variation is what allows the sensor to work with analog fingers in a digital world of computer chips.
What's more, you leave your fingerprints everywhere, thousands of times a day. Some of those fingerprints you're leaving right there on your iPhone, and on the home button itself. You don't write your passcode down on the back of your iPhone, do you? Now granted, not every print you leave is going to be useful, but it's not going to be that hard to find a clean one. Forensic scientists do all the time when they're investigating crimes.
If you want to provide evidence to a jury that somebody was there at a crime scene, fingerprints are a pretty good way of doing that. If you want to lock something, a fingerprint is terrible as a key.
I've said it before, and I'll say it again. Both police and criminals are very eager to see widespread adoption of analog biometric security in consumer devices. What they fear is digital security, the power of discrete math to protect secrets.
Pay transactions you have done so far is...? 
