Because Washington has a Constitutional provision requiring that no law shall take effect sooner than 90 days after the end of the session in which it is adopted [0] unless it is an emergency law passed with a 2/3 vote, and the common convention for most normal laws is to set the first January 1 certain to come after the 90-day period of the current session as the effective date so that "new law day" for non-emergency laws is consistent.

[0] Each state is different here, but a "90 days after end of session", or "90 days after passage" rule for the soonest a passed bill can go into effect, with exceptions for emergency bills with special rules including a supermajority requirement, are pretty common, as are conventions of setting a January 1 effective date in the legislation itself when the minimum is X days from end of session or passage.

One of those is even enshrined as an amendment: https://constitution.congress.gov/constitution/amendment-27/