2 Reasons I can think of.
1) They dont want every car hitting their servers at once. As the fleet grows, from what I read in articles and such, they arnt configured to use a cloud service like AWS that can grow exponentially like apple and android can do. They have their own Vmware Data center. So this could be to curb too many connections and cause a DOS.
2) As it is showing already, they want to do it in waves as an extra precaution to make sure the update is good. This allows them to evaluate a couple waves and make patches accordingly. Folks that have 39.6 have already been updated to 39.7, so this shows they stopped the fleet wide distribution to make some tweeks. And really, its a car that carries people at fast speeds. I am frankly happy they take a lot of precaution when updating. A car is not like a phone where a bad release could kill someone.