Because it's not feasible, nor cost effective. If every ADSL user was guaranteed their bandwidth on the international links 24/7, you're asking for a contention ratio of 1:1.
Considering ADSL users expect decent speed and latency, and assuming everyone has a 512k connection (and I want at least 4Mbps

) it means Telkoms (33% of 120gb) bit of the SAT3 cable could only support about 77343.75 ADSL users and no other traffic. We're already at over 100000 ADSL users at the moment, so it's not even physically possible.
Thus we have contention, and the only way slice up the bandwidth is by usage, to allow all users fair use of hte contended bandwidth.