All of Sony's U.S. Xperia devices that don't have fingerprint sensors, which is every phone overseas that does have a fingerprint sensor, have all come to market without them due to a business decision that Sony had to make, according to Sony U.S.' Don Mesa who recently spoke to Tech Radar and gave this response when asked. Although this doesn't exactly answer the question or give a reason why, Sony likely had their reasons for choosing to drop a certain feature when bringing the phone over to the U.S. smartphone market and if wasn't the fingerprint sensor it would have been something else.
While Sony is quiet on any really specific details for the choice made to leave the fingerprint sensor out, there could be many reasons, such as cost, which caused them to forego using it in the U.S. model. Of course, with Sony not stating as much this is purely speculation. According to Mesa, it was a business decision that was needed while they enter into the market of unlocked phones in the U.S. Normally, you would see Sony's phones sold at carriers, even though thy were few and far between, but with the Xperia X series and potentially other devices, Sony will be sticking to the unlocked phone market, and there is already loads of other unlocked devices available, so perhaps Sony had to leave off the fingerprint sensor for a business decision that would allow them to stay competitive.
Mesa also states that "a lot of factors needed to be checked off" before they could sell the phone in the unlocked market in the U.S., and this was one of those factors. So, as vague as the statement may be, Sony left it out for a reason. What that means for consumers is that they simply have to live without the ability to use the fingerprint sensor for authorizing mobile payments or unlocking the device if they want to go with a Sony phone, or choose a different device entirely to gain this functionality. It's also worth mentioning that other manufacturers use different hardware in devices for different regions too, such as Samsung with more than a few of their flagship Galaxy devices, or HTC with numerous devices in Asia. So while U.S. consumers may not like the idea of a Sony device with no fingerprint sensor, there is perhaps some peace of mind in knowing that other OEMs are doing the same thing elsewhere, and it all comes down to business and the markets they're selling these devices in.