HTC Sense 6 lock screen is now available in Play Store for selected HTC devices. HTC is making its apps available in Play Store for the past few months. Previously the Sense launcher, SenseTV, and the HTC Service Pack were made available in Play Store.
Putting stand alone apps in Play Store makes updating them easy. The company does not have to release a full firmware update just to update some of its apps. Like Sony made Xperia Keyboard available to Play Store so that they can update it easily through Play Store update.
The purpose of making stand alone apps available in Play Store is not to provide them on all phones. Like Xperia Keyboard is only available for Xperia phones. Similarly HTC Sense 6 lock screen app is available to selected HTC devices only. Any other Android device would not be able to get its taste. So now if HTC ever wants to provide something new in lock screen, they will push a stand alone update in Play Store.
You can download the HTC Sense 6 lock screen app from Play Store.