From https://en.wikipedia.org/wiki/1.1.1.1#Criticism_and_problems :

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.