Verizon works through a certification process with the OEMs before releasing a software update. Each carrier can take a certain amount of time to do this.

In the case of Android 6.0 - Many OEMs don't even have finalized customized images for their phones built yet. Samsung isn't slated to begin rollout until end of Q1 per their roadmap. Verizon will start to see those updates pushed Q1 to Q2.

If you need Marshmallow or the latest OS updates off the bat, go with a Nexus device. Those always get the updates first.