#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. 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.
This problem is already solved. Some wallets let you generate multiple Bitcoin addresses. And one can use mixers to enhance your privacy.