Because up until the turbo inlet you have a nice low pressure area that the turbo can use to grab as much air as it can.
3" water hose to a 2" spout doesn't work because either the water is under pressure and pushing it's way through or it's on the other side where the water is coming out into the 3" hose and cavitating because the 3" flows so much more.
Now say we have a fire hose hooked up to a hydrant with a nozzle on the end that has a turbine that will spin. Pressure forces water through the nozzle. What happens when we reduce that pressure? Not as much water flows out right? Keep reducing the pressure and pretty soon the turbine has to start powering itself to try and shoot water out instead of the water pressure pushing it's way through.
Take a 1ft section of garden hose and blow through it. Now take 50ft of the same garden hose and blow through it. The length requires are larger cross section to flow the same. So 3" right to the filter means only 1-2" of 2.25" at the venturi section of the turbo which helps to speed all that air into it.
Look at compound turbos. Two smaller turbos used to make tons of boost. Both set to a 2:1 pressure ratio aka 14lbs of boost. But what comes out of the 2nd turbo is 28lbs of boost in the intake. Why? because it got fed with 14lbs of boost at it's inlet and doubled it.
Same goes for your intake air filter setup on your turbo. If you can feed it atmospheric pressure (0psi) and you need 179k rpms to make 14psi you'll get 14psi out. Now you get full vacuum (30" of HG
or -15psi) in your intake tube. How fast does the turbo have to spin to make 14lbs of boost? Infinately because it get's no air. It might have to spin 300krpms if you allow it say 10" of vac in the intake tube. Have to figure it out on a compressor map
for exact #'s but I think you should get what I'm getting at.
The stock intake pipe on a VNT
flows more than what you currently have. Just because the inlet on the turbo is only 2.25" id doesn't mean that having the same size pipe isn't causing a restriction.