#1. security. When you make a bitcoin payment; that one payment address then forever connects you to your wallet thus allows the receiving person to see all your previous receives/sends, and, all your future receives/sends. Any payment you received, or spent, then crushes your anonymity. You are forever outed; fully exposed.
I think this is indeed the main feature of blockchain technology that satoshi introduced from the start. Anonymity is not the main / ultimate offering of bitcoin creation.
Really, it's no different than everyone in the world watching your bank account; seeing all the payments coming in and going out. Bottom line is you can never send or receive a payment to anyone in the real world. Your bitcoins/activity will forever be linked to you. Chainalysis perfectly lays out the problem.
Everyone can only track it, but bitcoin is still yours and only you can control it.