Lazy school admins
I discussed exactly this issue with a pal of mine the other day - he is a senior master at a very well known and well thought of school. My daughter was given this rubbish last year and has resulted in a wasted year of her time before we placed her in a college that would provide the course SHE wanted.
My pal's comment was that timetabling is a problem, but if a school puts sufficient effort into it - and ironically a bit of computer time - they can - and should - achieve a satisfactory (not perfect mind) outcome for ALL students.
Simply letting the "gifted" pupils have their options and letting the rest squabble over what is left is pathetic.