HTC's failure should be a lesson to Nokia. Carrier exclusivity is a bad idea.
Samsung was able to put their flagship product on all 4 US carriers while maintaining their Galaxy S3 branding. HTC, on the other hand, decided to put their flagship product on one exclusive US carrier. The results were predictable. How could HTC expect to compete with Samsung or get any meaningful sales number compared to the S3.
Here comes Nokia following right in HTC's footsteps. The results are clearly predictable.