Samsung actually has a very good reason. They are still troubleshooting their foundry processes for the Exynos chip, which is creating a lower yield than anticipated. You can see that there is still a supply issues with creating enough chips for the European market, and they launched in May. That is why Samsung put the Tegra 2 chips in for certain smaller markets even though the Tegra 2 costs more for slower performance and higher heat.
Since the US market is huge, if the haven't figured out how to fix their Exynos process yet, there is no way to supply enough chips for the US market while still sustaining their European and Asian regions, which are theoretically more important to Samsung. Unfortunately, that means there's a chance we may get a Qualcomm, TI, or Nvidia chip in our phones.