Easy. Deep packet inspection (DPI) working very well for censorship in many countries. Sometimes authorities in my country even block masked/obfuscated traffic through Shadowsocks etc, and no matter traffic is coming from known VPN provider or your rented private server. 

But at such moments the infrastructure for delivering regular traffic is usually disrupted, and encrypted VPN is still needed for business purposes inside the country, so the authorities do not block Shadowsocks and similar protocols completely yet. 

Such protocols as OpenVPN, L2TP, IKEv2, WireGuard aren't working in my country right now. Sometimes for someone WG is working, but not for me. I also noticed that mobile internet providers block Shadowsocks more often and for longer periods than 'wired' internet providers. 

By the way my VPN provider created a special page for users from my country, where it tells how to switch to SS and Outline.