HTC One (M8) for Windows might have debuted sooner were it not for Microsoft acquisition of Nokia

When Microsoft loosened its manufacturing requirements to allow companies to use existing hardware designs and run Windows Phone, the HTC One (M8) was the perfect reference to build off of because of those BoomSound speakers, top shelf internals, and slick build quality.
HTC was on the same page it seems with its One (M8) design, but it held back the HTC One (M8) for Windows due to the merger with Microsoft and Nokia Devices and Services. “We were waiting to see how the relationship would take shape,” according to HTC USA head Jason Mackenzie, “We weren’t sure what their strategy was going to be.”
Any tension was relieved when Microsoft announced its restructuring which is going to involve 18,000 layoffs from the company, more than 12,000 of which will be from the recently acquired Nokia unit. Given that, Microsoft needs a high-quality device like the M8 in the stable.
That said, HTC did see a gap in high-end hardware for Windows Phone. “Nokia offers terrific hardware, but HTC is just more established in the US in terms of momentum,” Mackenzie said of the M8, “[it is] the first time a smartphone manufacturer has launched an iconic device on multiple operating systems without making any compromises.”
sources: CNET via WMPoweruser