As Centaurus has said, T-VASIS are more expensive to maintain. Additionally, I am told that spare parts are no longer available. And if you were to install a new T-VASIS, there are ten boxes per side against the PAPI with four per side and hence considererably more expensive to install and calibrate. Flight checking both aids is almost the same procedure. Note that in Australia a number of PAPI's have never been flight checked.
So essentially the cheaper item now rules the market place!!
This is an interesting thread - I hope to see some positive outcomes from the ideas being put forward.
Blackburn