Good question. These are Windows RT tablets, which won't be getting the Windows 10 upgrade. They have additional hardware features in them to support driving external monitors that the current Windows phones don't have. This is why new technology is needed to power Continuum. Some of the current phones can output things to Miracast / USB, but this is different to what is needed for it to be able to run an independent monitor (and scale up applications) and still have the phone as fully usable for other tasks like incoming calls, SMS, etc. When the tablets were designed, I imagined video output was considered necessary and taken into consideration, but clearly this wasn't the case with the phones as Continuum is a new concept for Windows Mobile.