GOV.UK Design System projects is designed to work with a distributed ecosystem that is the UK Government. Even if we wanted to we can not force a certain framework or approach on teams distributed in different departments across the UK.
With this in mind we need to support the lowest denominator and be technology agnostic.
Any approach that relies on HTML being identical on both the client and server will mean our JavaScript will only work with a framework that we have chosen. (See virtual dom hydration)