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Pascal's principle says that
the pressure applied at one point in an enclosed fluid under equilibrium conditions is transmitted equally to all parts of the fluid
and thus the pressure in e.g. a hydraulic jack is equal everywhere.
However, in a stream tube, the pressure at the narrow end will be greater than the pressure at the wider end. Therefore, Pascal's principle does not apply to a stream tube and we need to use Bernoulli's equation to model the stream tube phenomenom.
Pascal's Principle applies only to confined/enclosed fluids. When fluid is flowing through a stream tube, it is not completely confined (the tube has two open ends, after all; if it didn't, then steady flow wouldn't be possible).
