Because a link should get a /64 as then it can use autoconfiguration using this /64, which specify the first 64 bits and the EUI-64 portion of the link, totalling into an 128 bits IPv6 addres. With a /48 you would have 64-48=16 missing bits. The /48 you get is supposed to be divided over 65536 subnets.

Because subnets are /64 ... so with a /48 prefix you can have 65535 subnets. eg you have 2001:1:1: 0-ffff: 2:2:2:2 your prefix subnet-id host-part eg you could announce 2001:1:1:0 on eth0 and 2001:1:1:1 on eth1 and have 2001:1:1:ffff as a "protocol translator" where you map in the whole ipv4 address range :)