simply the numbers and the purposes of most visits.
I have talked to wardens who refuse to visit the piers unless they are part of a task force (that are occasionally set up). Stopping a private boat with a couple of anglers, or visiting a Sportfishing boat, is different than a lone warden visiting a pier full of anglers who may or may not be cooperative (and may be somewhat intimidating).
In addition the wardens are often checking for fishing licenses. Thus they might visit the Paradise Cove Pier in Malibu (which is private and requires a license) while not visiting the nearby Santa Monica Pier which does not require a license.
I have talked to wardens who refuse to visit the piers unless they are part of a task force (that are occasionally set up). Stopping a private boat with a couple of anglers, or visiting a Sportfishing boat, is different than a lone warden visiting a pier full of anglers who may or may not be cooperative (and may be somewhat intimidating).
In addition the wardens are often checking for fishing licenses. Thus they might visit the Paradise Cove Pier in Malibu (which is private and requires a license) while not visiting the nearby Santa Monica Pier which does not require a license.