The short term answer is because of forum spammers.  Odoo has suspended some karma gaining features until they can implement new spam filtering features in the forum.

For now you need to earn the karma by asking really good questions (like this one) or by helping others answer their questions and gain votes or have your answer accepted.

You get fifteen karma points for your answer being accepted and two karma points for accepting someone's answer to your question.

Sorry for the short-term inconvenience.