Gee Muffinman I thought a man of your great knowledge and life experience would already know this stuff!
Only reason I know all this is that I was asked it in a interview and had no idea at the time!
The answer is Three fold.
One being international standardisation. Everywhere else in the world use PAPI and so therefore we have to as well.
Two being they are expensive to maintain due to the fact that T-Vasis run up to 20 lights at a time which makes for a hefty power/maintenance bill. Also installation is tricky as it requires alot of surveying and ground work.
Thirdly after the RFDS accident in Mt Gambier there were some issues raised in regard getting false indication when in areas of visible moisture and fog.
So the real answer is that it's a cost saving issue however Darwin airport will be claiming that it will be improving safety and bringing themselves into line with world's best practice. Even though T-Vasis was considered a superior system when it came out, so much so that there was a push to make it the world standard at the time.
It was also an Australian invention!