register_activation_hook() only attaches a function to run on activation of your plugin, not on updating. See the docs for full details, particularly this part:
3.1 : This hook is now fired only when the user activates the plugin and not when an automatic plugin update occurs (#14915).
Of course, you could force your hook to run by deactivating and re-activating your plugin, but you certainly don’t want your users to have to do this 🙂
A better way would be to manage the ‘database version’ of your plugin – so perhaps, store the ‘current version’ number of your plugin in the database. When your plugin runs, check this version number against the real version of your plugin. If it is ever different… that’s when you want to add this field to the database.
Of course, for brand new installs of v1.1 of your plugin, you still want to run this as you are currently. You just need to also consider the upgrade path that existing users – such as yourself in this instance – will take.
Further reading which I would definitely recommend for this topic: