Maybe the actual performance of the image stabilization wasn’t good enough because the SoC couldn’t keep up with erratic movements. Maybe they were worried about background tasks being held up from the processing that the SoC has to do with image stabilization enabled. I simply think that Google did some performance tests with the feature on the Nexus 5X and for whatever reason they felt that it just wasn’t good enough. They are more interested in making profit from you using Google services on a Nexus device than actually selling them.
Google uses the Nexus program to showcase what they feel Android should be and that is as far as it goes. I feel that since Google sells so few Nexus devices that they aren’t motivated by profit from hardware like this. The cynic would tell you that Google just wanted to create a disparity of features between the Nexus 5X and the Nexus 6P to entice more people to buy the more expensive device. Like I said, there could be a number of reasons as to why Google decided not to put this feature in.