The CAA reasoning behind the 28 day limit is pretty clear; they don't want foreign-registered aircraft to be 'based' in the UK - it's seen as an abuse of the ECAC agreement. (Imposing a 28 day limit is also an abuse of the agreement!).
As the CAA is a private company with a requirement to turn a profit, how can they justify simplifying the requirements for home built aircraft as it would hurt their bank balance? They know that no-one would bother with the UK register and everyone would jump to less onerous jurisdictions. You only have to look at how many certified FAA 'N' reg machines knocking around to know that to be the case. The CAA can't control the circulation of certified machines, but they sure as hell can control the amateur built stuff.
The official line will doubtless be about 'safety'; but the real reason is that if it's too simple (read cheap), there won't be any money to be made out of it.
JJ