It's a protocol layer thing, mostly down to "what prefix sizes can I reasonably expect other providers to accept?"

For IPv4 the answer was a /24; for IPv6 the answer seems to be a /64 at this point, and burning an entire /64 to get a couple of DNS servers (the typical use case for anycast) to resolve slightly faster is overkill. It's also probably a great way to annoy your RIR when they ask 'how the hell did you burn through your initial IPv6 delegation with only two hosts' ;)

EDIT: I meant /48, not /64! Whoops-a-doodle.