Why Does Vmo decrease with Altitude

[concordino]

I am looking at B1900D flight training manual and it says:
Vmo is 245 from Sea level to 13500 feet
and decreases to 195 from 13500 feet to 25000'
Why is that so ?

many thanks in advance

[Squawk7777]

The King Air 200's ASI has a Vmo bar that does change with altitude. That is a basic form of a mach meter. I am not sure about the 1900's ASI.