It's because of two things: OEMs wanted Microsoft to help them sell some new devices (minimum requirements for Windows haven't been raised for the most part for 13 years. If your machine could run Windows 7, Windows 10 worked fine), and because the cancelled Windows 10X project (because every time Microsoft claims they've figured out how to compartmentalize Win32 apps they kill it internally) had a brand new sparkly interface that could be backported without a ton of extra work.