According to Sony, it is because they are still working on updating the necessary drivers to ensure that everything will continue running smoothly post-update. In a post on Sony’s support page (via Microsoft News), Sony notes that, “To ensure your VAIO PC works with Windows 10, it is strongly recommended that you wait to upgrade your PC until the drivers are ready.”
The company adds, “We are testing supported models and will be providing the results as quickly as possible. We plan to release Windows 10 upgrade information and, for Windows 8 and 8.1 models, drivers, applications and detailed information on the schedule below based on the operating system originally installed on the PC.”
According to the support document, they note that these drivers are expected to be updated in October/November 2015, depending on which version of Windows was preinstalled on your computer. Once that’s done, you should be able to update your computer to Windows 10 just fine.