> I don't understand why the ban cannot come into effect immediately.

This is the default in Californias constitution:

> Except as provided in paragraphs (2) and (3) of this subdivision [Article 4, section 8], a statute enacted at a regular session shall go into effect on January 1 next following a 90-day period from the date of enactment of the statute.

There are additional legislative hurdles to enact legislation faster than that.