They are not durable because when durability was added to PostgreSQL a long time ago people were not sure how to add durability to hash indexes, so that task was left for later. But later did not happen, until very recently when some people worked hard on adding durability for them. Adding durability required quite major changes.

PostgreSQL 10 which will be released this autumn will include durable hash index, do not use hash indexes until then unless you really know what you are doing.