It's due the the design of the turbine housing, with an evo turbo, the wastegates are separated from the exhaust flow by a wall in the casting.
Mistubishi Evo turbo
It's because of this, that the spent exhaust from the turbine wheel, and the exhaust bypassing the turbo through the wastegates are able to take different paths.
Subaru Impreza WRX STi turbo
you can see here, the exhaust from the turbine wheel and the exhaust from the wastegates are combined into one enclosure, which is why there are no "O2" dumps on subaru's using the stock USDM single scroll turbos.
But there is hope to do this on a subaru, it is with a JDM twinscroll style turbo.
Moore performance makes a divorced downpipe with a "O2" dump for the twinscroll turbos.