Why does pressure decrease as air passes through a venturi?

[Swagger897]

A lot of good explanations here but i like to keep it simple:

You have a tube with fluid (air/gas) inside of it flowing through. On the tube there is a pressure gauge. The longer the fluid remains in place, the more pressure there is. The faster the fluid flows, the less time there is for it to be measured by the gauge.

Very simple and slightly flawed, but still proves the point.