Why are my rules not restored when I launched Chromium again?

By design, whitelist and blacklist rules are temporary by default. In order to make your rules permanent, you must locked them down:

This is by design. At first, this may feel bothersome when you need to create all the set of rules for the web sites you trust completely. Eventually, soon enough, once all your basic set of rules is created, you will be happy that rules are temporary by default.

This way you won't end up bloating your ruleset for the common case of just unblocking a web page for which you don't want to permanently whitelist.