The answer lies in what it costs to provide that NBN connection to you. The Federal Government always wanted the NBN Co. to be a commercial operation, it had to be profitable, and both Labor and LNP wanted this.
The ADSL connection is fairly cheap to run from an ISP (RSP in NBN speak) point of view as it is a very mature tech and initial costs have long been paid. NBN on the other hand has very new implementation costs and the on-going fee structure reflects this.
For a list of what an RSP must pay the NBN Co. for each connection visit the NBN Wiki thread here:
Some of these costs are one offs then others are on-going monthly costs. Recently CVC costs have again dropped but they are the biggest on-going cost an RSP has to pay. When the connection is made by the new NBN to users, the user is helping pay off that infrastructure cost. This means that the expense is high currently and will remain so for some great while to come (until at least those implementation costs have been recouped by NBN Co.).
There is an emerging argument that NBN Co. should not be a commercial operation and rather should be run as an essential Govt service and have it’s implementation costs paid out of General Revenue and not recovered by having consumers pay for the use (they will of course pay but it will be a hidden cost in taxes). If this approach was taken many of the on-going costs could be even further substantially reduced leading to a lower per user payment.
As others have answered it is the MTM NBN which has led to many issues. The copper your ADSL relies on in many instances will still be relied on for the final part of the connections to premises. This copper is aging and failing so will be a further expense to remedy sooner or later. The MTM part of the NBN creates complexity because of the various tech used to attain it. This complexity increases service/maintenance costs as specialist people have to be retained for each variance of NBN connection. These are costs that RSPs must reimburse the NBN Co. for and if they have to reimburse then the end user has to reimburse the RSPs.
There is a benefit but it is very much lost in the chaos MTM has unleashed on us. This is why we should as a population be pushing hard for the FTTP outcome (and there will be additional costs to implement this now). FTTP for the vast majority will provide a much more resilient, robust, and cost effective way of connecting our nation. There will be some small percentage because of placement who will still require Satellite and Fixed Wifi for some time to come but they will at least enjoy connectivity that was previously impossible for them to achieve (long term they to will be FTTP).