The only good reason I can think of is that the UK had a very tight focus on subjects: at GCSE you study 10+ things, at A(s) level 3-4, at university 1.5. You can't really use GCSE results for a UK degree entry because 80% of those subjects are irrelevant for this degree because (unlike EU\North America) a (say) physics degree has zero credits for English lit or history or even chemistry. A Level results are too late. So that puts everything on AS results (and I think the Tories got rid of the actual AS exams a few years back?).

I am a big "final exams not coursework please" guy. But they'd be better having end of term exams for the 6 terms in A Levels and using the results from the first 5 for most of admissions IMHO.