At the most fundamental level: Random. Tesla routinely pushes out newer firmware to a smaller audience of randomly selected cars (call it beta testing, A/B testing, etc etc). This can go on for weeks with a new build every week, until some point where they start a more massive rollout (those are usually 1/3 or 1/4 of the population per day).
The only exception is: If you book a service appointment and bring your car in, that sometimes triggers the most recent build to be installed on your car.
But for most people: Don't feel left out that your car is not getting updates that exist out there. Unless you find yourself left out of a major rollout, there's nothing wrong with your car. You can use either ev-fw.com or firmware.teslafi.com to get a sense of the distribution of updates. Right now, the major rollout is still 2017.34. There was a minor and no-longer-active rollout of 2017.36 that reached a small population, but since then, 2017.38 and 2017.38.4 have been very small rollouts.
