The reason we don’t automatically update it is because you could have two setups for the same spec. For example, you might make a Raid and Mythic+ setup for your Unholy Death Knight spec with very different talents and major essence.
You typically don’t want the talents and major essence chosen for a setup to change every time you import – they have a big impact on what gear you will choose. When we did automatically update (before we allowed you to make multiple setups), we also found that people mistakenly were not setting talents properly before optimizing. For example, you might take a talent that is fun or convenient for going around doing daily quests, but you never use it in raids and don’t optimize around it. People would often update their character and optimize, forgetting to change that talent back to their raid talent before optimizing.