Supposedly because Cloudflare doesn't send EDNS client subnet information. [1] is archive.today's statement, it has been discussed at length (with much head scratching) on HN in the past.

1: https://twitter.com/archiveis/status/1018691421182791680

Because CloudFlare is not passing along client IP/subnet to the authoritative nameserver, so archive.is nameserver refuses to respond (or responds with bogus results). archive.is justifies it with the need for geo-aware DNS resolution, but i personally think that's the very wrong way to do it.

Good connectivity is determined by your ISP knowing what best routes to take to a certain IP, and the DNS is not aware of this BGP layer. Announcing your subnet from several points of presence enables client ISPs to choose the best route for them (cheapest/fastest), but resolving to different IPs depending on client "location" may provide inconsistent results that don't go over the fastest route.