Because it wasn't popular on any other device. Apple may not release the best technology, but iWhatevers are popular. As soon as people begin to accept fingerprint authentication for iTunes, they'll be ready to accept it for credit card purchases, to check out books at the library, to pass through airport security, at the dentist, when voting, etc. Then we will begin to see the real problem with fingerprint authentication, which is this right here.
@xof: Yes, as long as the second factor isn't my mother's maiden name or the street where I grew up.
@David_Diamante and everybody: This starts to run off-topic just a little, but it doesn't matter what the environment is, or what you're keeping secure. It's your stuff and you shouldn't want anyone else just looking at it [banana] without your explicit permission. Well, I don't, anyway. I don't play candy crush, or use mytracks, but I do have pictures of my friends and family on my phone, and I don't want just anybody looking at those pictures. To put it all back on topic, if my data is less secure because I'm using a weak method of protection, then there is an increased risk that someone will get at my stuff. I'd rather use my own version of correcthorsebatterystaple than my fingerprint or a 4-digit (or even 10-digit) PIN. They are weaker, and in the case of my fingerprint, if it's copied, I can't change it.
And we can't increase our security through fashion (gloves) because we've been leaving our fingerprints everywhere since we were little (ask your mom if you don't believe it).