Why is UCEPROTECT so influential?

I have a question for the spamosphere: Why is the tiny UCEPROTECT blocklist so influential with large telecommunications providers? Nearly every provider I speak to is enormously concerned about their reputation on the UCEPROTECT network. This concern seems strange to me, considering the small penetration of UCEPROTECT versus other blocklists like Spamhaus, SORBS, and Trend Micro MAPS+.

Reviewing millions of delivery attempts at a large service provider, I found literally zero references to UCEPROTECT in SMTP error messages. Here is a summary of the most popular blocklist service URLs, found by searching the outbound SMTP filtering logs of an ISP and a major cloud hosting provider:

As you can see – there’s no UCEPROTECT to be found. In fact, I grepped the logs of millions of SMTP connection attempts from many different networks, and could not find examples of receivers indicating that UCEPROTECT was the reason that a connection or message had been blocked.

So, then, my question to you all is: Why is UCEPROTECT taken so seriously? Because, they definitely are.