The MAP sensor tells the ECU when the required boost is reached. If the MAP sensor can't read the boost then how can it report to the ECU correctly?
Let's take an example:
On a standard map, with a standard Map sensor (for arguments sake, let's say peak boost is 20psi and the MAP sensor reads up to 25psi) then the MAP sensor reads the boost all the way up to 20psi and reports this figure to the ECU. That's all it does. It doesn't control the boost in any way, just reports the actual boost to the ECU. Once the ECU gets the 20psi signal from the MAP sensor it controls the turbo vanes to stop overboosting.
On a remapped ECU with the same standard MAP sensor (let's say peak boost is now 26psi and the MAP sensor reads up to 25psi) then the MAP sensor reads boost all the way up to 25psi and then stops. After that, all it can do is report to the ECU that 25psi has been reached, but the ECU is requesting 26psi, so keeps the boost on, waiting for the required figure from the MAP sensor. A figure that it's never going to get because the MAP sensor has already topped out at 25psi and can't say anything else.
Remember, the ECU relies on the MAP sensor to tell it how much boost there actually is, and then controls the turbo to limit boost to the map settings. Eventually it'll overboost so much that the ECU will decide that the MAP sensor is failed due to no change in the reading, despite calling for maximum boost, and it'll decide there's a
problem and shut down.
Basically, once the MAP reaches it's ceiling then the two stop talking to each other.