Why do you think tagging is a thing? It’s a way for gangs to mark their turf, plain and simple. Tagging means gang activity, which means violent crime.
Its not plain and simple. As someone who has talked to people who actually do it, it’s much more common to be art-related and marking that someone exists than anything gang related. Maybe it used to be a certain way but it certainly isn’t strongly correlated now.
And your assertion about the LA crime rate shrinking for years is incorrect: https://www.laalmanac.com/crime/cr01.php
In fact, violent crimes reached their lowest level around 2013, and have been steadily on the rise since then.