I'm wondering how it happened as well (and I was here at the time!).

From what I can reconstruct..

a) there were big petitions by the religious groups to introduce it as a voluntary system.
b) there was disquiet but no organised opposition from the teacher's union et al.
c) it was proposed by a left-wing minister for education
d) the department legally interpreted "may" as "must"
e) now it's actually implement the unions et al are kicking up a fuss.