I presume you mean that there are plenty of Biz seats available for purchase but when looking for redemption award seats (using points) there are only Economy class seats available?

If so, then you have discovered how QF manages inventory. There may be seats released closer to departure. You can also buy an Economy seat (not the cheapest classes) and waitlist for an upgrade when the WL window opens at 90 days (this will be determined 24 hrs prior to departure).

Not a great system, but it forces people to buy Biz or First seats. QF is very profitable (or at least it was last year).