It's because there is a shortage of secondary teachers, hence the financial incentive. However, as there is not a shortage of primary teachers, but a surplus, then the higher grants are not necessary. Same goes for training in Scotland at any age group - no bursaries at all as they have enough teachers, even secondary.
However, you say you got no support at all? When did you train, as primary PGCE students DO get a £4000 grant. Not as helpful as my £9000 for MFL, but still gives you a start. And remember, we have £3000 tuition fees now; if you trained a few years ago that wouldn't have been the case.