Why is fingerprint lock considered less secure than pattern lock on Nexus 5X?
The weakness is actually in the fingerprints themselves and not the way Nexus 5x implements it.

Fingerprints are less secure because they cannot be changed like a password and cannot be revoked if compromised.

Fingerprints are also left everywhere such as, at a glass. They can then be captured and reproduced. Think about this: Android uses a touch screen. The fingerprints are left on the screen after device's usage.

With a fingerprint it's almost good then it is good enough. Because the way it is touched or if you have a cut. Making fingerprints not hashable. Compared to a pin which has to be perfect every time.

They can be taken without permission. Not only from being left behind but also legally without consent. If a password is in your head it must be given, thereby, also leaving your right to privacy and your right to remain silent.

In the United States a person is also protected from testifying against themselves by the 5th amendment. Legally, fingerprints are considered as physical objects because they exist in the physical world. So they are not protected by the 5th amendment.