Probably because it was marginal, and the new enhanced software for Touch ID drove it over the edge.
If it's software why doesn't my phone have the problem?
And if its software why does replacing the phone fix it?
I've seen well under 100 reports of this problem since iOS 7.1 was released.
And why aren't there thousands of posts about it, as there are when real bugs are found in a release, as there were for this, with over 12,000 posts: iPhone 4s Battery Life? And even that one affected only a small percentage of users.
There have been a steady stream of reports of Touch ID failures since the 5S first came out, so it is a somewhat touchy technology (no pun intended). Apple has been replacing phones for this failure pretty steadily.