Technological websites noted that by using 1.1.1.1 as the IP address for their service, Cloudflare created problems with existing setups. While 1.1.1.1 was not a reserved IP address, it was and is used by many existing routers (mostly those sold by Cisco Systems) and companies for hosting login pages to private networks, exit pages or other purposes, rendering the use of 1.1.1.1 as a manually configured DNS server impossible on those systems. Additionally, 1.1.1.1 is blocked on many networks and by multiple ISPs because the simplicity of the address means that it was previously often used for testing purposes and not legitimate use. These previous uses has lead to a huge influx of "garbage" data to Cloudflare's servers.
That’s intentional, from what I remember. All non-DNS traffic is analyzed for research purposes (not by Cloudflare though).
A wake-up call for all those (ab)users of public address space is also desperately needed. All IPv4 addresses will soon be allocated. Failure to use only private address spaces will cause problems, very soon.
CloudFlare likely did this on purpose, because so many people can't get their heads out of their own asses and follow spec. Now there's a big spotlight on the people purposefully breaking the network. And it will be fixed, eventually, whereas previously, AT&T would have just said "take a hike".
Technological websites noted that by using 1.1.1.1 as the IP address for their service, Cloudflare created problems with existing setups. While 1.1.1.1 was not a reserved IP address, it was and is used by many existing routers (mostly those sold by Cisco Systems) and companies for hosting login pages to private networks, exit pages or other purposes, rendering the use of 1.1.1.1 as a manually configured DNS server impossible on those systems. Additionally, 1.1.1.1 is blocked on many networks and by multiple ISPs because the simplicity of the address means that it was previously often used for testing purposes and not legitimate use. These previous uses has lead to a huge influx of "garbage" data to Cloudflare's servers.