I
I don't know why they just don't do a big bang (you have the option of not accepting), gather up whatever bugs are reported into a post rollout bug fix and get things done.
Main reason is almost certainly there is scope for bugs that have major impact - either $ cost or reputational damage. There have certainly been bugs in the past that required service center/ranger intervention to get cars going again, and bugs that caused physical damage (12V battery charging bugs -> new 12V battery required). It's not beyond the bounds of possibility that they fail to hit in beta testing some combination of factors that affects (say) 10% of cars. Suddenly disabling 10% of the fleet would cause massive problems - so even though it's not likely, only a remote possibility, it is reasonable for them to be cautious.